In some traditions their philosophy says that concrete things don't really exist, only in the mind like just an experience. But is that vision important, or could just your relation to those things as if they don't exist be enough? In other words, is this philosophy just a way to concentrate on the spirit, instead of the outer world, or do they really mean this?
But is that vision important, or could just your relation to those things as if they don't exist be enough?
I like the description of the "South Indian Monkey Trap".
The parable is that one way to trap a monkey is to put some food (a nut or a banana) in a narrow-necked jar. If a monkey comes, it reaches into the jar, and grasps the food in its fist. Now that they've made a fist with food in it, their hand is now too big to remove from the narrow-necked jar. So the monkey is stuck in the jar. The villagers are coming. What general advice can you give the monkey?
Well, some general advice might be that you have to re-evaluate the importance of grasping the food. Is that grasping food more important than freedom, for example, more important than letting go?
Another bit of advice is that you might be panicking about people coming to get you, but if you pay attention to the sensations in your body, maybe sensations in your hand or wrist, is there anything you could learn from that?
The teaching isn't entirely that "concrete things don't exist", it's that our ideas about things are a result of a mixture of different things: including sensation, contact, consciousness, and attachment/grasping -- and the 'importance' that we place on something might be unwisely disproportionate to whatever values or properties may be inherent in the "concrete thing" itself.
In other words, is this philosophy just a way to concentrate on the spirit, instead of the outer world, or do they really mean this?
I'm not sure where there is "the spirit" as such. That might be an idea (about 'soul') that's slightly absent from Buddhism, though some traditions do talk about a Buddha-nature which might be similar to 'spirit'.
It might be better (or more usual) to say 'mind' instead of 'spirit'.
Still you mentioned "in some traditions": so maybe you'd be interested in this topic, Do Cittamatra / Yogacara explicitly refute the existence of an external world?
Whatever that exist outside we experience through our sense doors and paint a picture in our minds. This metal model we construct will not exactly correspond to what is outside will not the exactly the same from person to person depending on many factors like cognitive ability, sharpness of senses, etc. Hence that we know about the world is not perfectly in touch with reality and not always exist as we expect it to be. So it would be safe to say that anything we perceive as such does not exist in such a form that we perceive or expect it to be in. Also our outlook of the world is mentally constructed and does not exist in any form we expect or think it to be in. Also we have no ability to exactly know what exactly the world is as we are limited by cognitive ability and or faculties. So we will never know the world as it is, perhaps unless you become a Buddha as only Buddha can see all phenomena.
"I will teach you the All. Listen & pay close attention. I will speak: Sabba Sutta: The All"