According to the Kimsuka Sutta:
“So too, bhikkhu, those superior men answered as they were disposed in
just the way their own vision had been well purified. “Suppose,
bhikkhu, a king had a frontier city with strong ramparts, walls, and
arches, and with six gates. The gatekeeper posted there would be wise,
competent, and intelligent; one who keeps out strangers and admits
acquaintances. A swift pair of messengers would come from the east and
ask the gatekeeper: ‘Where, good man, is the lord of this city?’ He
would reply: ‘He is sitting in the central square.’ Then the swift
pair of messengers would deliver a message of reality to the lord of
the city and leave by the route by which they had arrived. Similarly,
messengers would come from the west, from the north, from the south,
deliver their message, and leave by the route by which they had
arrived.
“I have made up this simile, bhikkhu, in order to convey a meaning.
This is the meaning here: ‘The city’: this is a designation for this
body consisting of the four great elements, originating from mother
and father, built up out of boiled rice and gruel, subject to
impermanence, to being worn and rubbed away, to breaking apart and
dispersal. ‘The six gates’: this is a designation for the six internal
sense bases. ‘The gatekeeper’: this is a designation for mindfulness (sati).
‘The swift pair of messengers’: this is a designation for serenity (samatha) and
insight (vipassana). ‘The lord of the city’: this is designation for
consciousness. ‘The central square’: this is a designation for the
four great elements—the earth element, the water element, the heat
element, the air element. ‘A message of reality’: this is a
designation for Nibbāna. ‘The route by which they had arrived’: this
is a designation for the Noble Eightfold Path
Samadhi and samatha are related words. You practise samatha (serenity meditation) to achieve samadhi (concentration).
The ATI glossary defines samadhi and vipassana as:
samādhi: Concentration; the practice of centering the mind in a single
sensation or preoccupation, usually to the point of jhāna.
vipassanā: Clear intuitive insight into physical and mental phenomena
as they arise and disappear, seeing them for what they actually are —
in and of themselves — in terms of the three characteristics (see
ti-lakkhaṇa) and in terms of stress, its origin, its disbanding, and
the way leading to its disbanding (see ariya-sacca).
So the message of the Kimsuka Sutta is that you establish mindfulness (sati) which then needs BOTH serenity (samatha) and insight (vipassana) to show reality clearly.
I would use the analogy of a camera. There are three parts of a camera you must control to take a photo.
First, you open the camera shutters to allow light to come in. That's mindfulness (sati).
Next, you have to control the zoom of the camera, while simultaneously moving the camera around and finding the right subject.
The zoom setting is the concentration (samadhi). Adjusting the zoom setting is serenity meditation (samatha).
Moving the camera around to find the right subject is insight (vipassana).
If you concentrate on the wrong subject, you will not get the photo that you originally intended to get. You want to take a photo of the sunset, but instead you zoom onto a tree, then you would see a tree clearly.
When you point to the right subject, but if you don't zoom correctly, you would see the right subject, but not clearly. You point your camera to the sunset but you did not correctly adjust the zoom, so you would inevitably see a blur image.
The main point of the Samadhi Sutta is not about seeing things, but it's about seeing things CLEARLY.
The Pali version of the sutta says: "Samādhiṃ, bhikkhave, bhāvetha. Samāhito, bhikkhave, bhikkhu yathābhūtaṃ pajānāti."
"yathābhūtaṃ" means "in truth; in reality; in its real essence."
"pajānāti" means "knows clearly".
So, without concentration, you cannot see reality CLEARLY.
Use concentration to see reality CLEARLY (pajānāti).
Use not only serenity (samatha), use BOTH serenity (samatha) AND insight (vipassana). With vipassana, you see reality, but with vipassana AND samadhi, you see reality CLEARLY.
From the Samatha Sutta:
"If, on examination, he knows, 'I am one who achieves internal
tranquility of awareness but not insight into phenomena through
heightened discernment,' then his duty is to make an effort for the
maintenance of internal tranquility of awareness and for insight into
phenomena through heightened discernment. At a later time he will then
be one who achieves both internal tranquility of awareness and insight
into phenomena through heightened discernment.
"But if, on examination, the monk knows, 'I am one who achieves
insight into phenomena through heightened discernment but not internal
tranquility of awareness,' then his duty is to make an effort for the
maintenance of insight into phenomena through heightened discernment
and for internal tranquility of awareness. At a later time he will
then be one who achieves both insight into phenomena through
heightened discernment and internal tranquility of awareness.
"But if, on examination, the monk knows, 'I am one who achieves
neither internal tranquility of awareness nor insight into phenomena
through heightened discernment,' then he should put forth extra
desire, effort, diligence, endeavor, relentlessness, mindfulness, &
alertness for gaining those very same skillful qualities. Just as when
a person whose turban or head was on fire would put forth extra
desire, effort, diligence, endeavor, relentlessness, mindfulness, &
alertness to put out the fire on his turban or head; in the same way,
the monk should put forth extra desire, effort, diligence, endeavor,
relentlessness, mindfulness, & alertness for gaining those very same
skillful qualities.
"But if, on examination, the monk knows, 'I am one who achieves both
internal tranquility of awareness and insight into phenomena through
heightened discernment,' then his duty is to make an effort in
maintaining those very same skillful qualities to a higher degree for
the ending of the effluents.