If access concentration, or at most the first jhana, is the requirement for successful insight meditation what is the purpose of the other form and formless jhanas?
Although the higher jhanas are not required, they're strongly encouraged by the Buddha evident in the high frequency with which they are mentioned throughout the suttas. Ven. Gunaratana in his "The Jhanas" wrote:
The Buddha is constantly seen in the suttas encouraging his disciples to develop jhana. The four jhanas are invariably included in the complete course of training laid down for disciples. They figure in the training as the discipline of higher consciousness (adhicittasikkha), right concentration (sammasamadhi) of the Noble Eightfold Path, and the faculty and power of concentration (samadhindriya, samadhibala). Though a vehicle of dry insight can be found, indications are that this path is not an easy one, lacking the aid of the powerful serenity available to the practitioner of jhana. The way of the jhana attainer seems by comparison smoother and more pleasurable (A.ii,150-52). The Buddha even refers to the four jhanas figuratively as a kind of Nibbana: he calls them immediately visible Nibbana, factorial Nibbana, Nibbana here and now (A.iv,453-54).
The formless jhanas are useful for understanding later stages but all require at least first jhana to cultivate successfully. In some circles they are considered recreational or dangerous to dwell in unnecessarily. Remember that the Buddha mastered these states and declared them all incomplete.
According to Daniel Ingram in "Mastering the Core Teachings of the Buddha" and many others, only access concentration is required to have a reasonable chance at success with insight practice.
1st jhana is reccommended though because this is a stabilized form of access.
Even though this is true, it doesn't change the fact that the 4th jhana is true concentration.
Asking what the point of true concentration is... is like asking "what is the point of perfect, clean water? It is just more skillful and beneficial.
The 1st jhana, 2nd, and 3rd are inferior and have flaws.
The 1st jhana is fuddled and filled with coarse thoughts and effort--relative to the other jhanas. It is still a supermundane release far beyond the scope of the Desire Realm, beyond anything any typical person can experience in their lifetime.
The 2nd jhana, although filled with unsurpassed joy and unprecedented effortlessness, is still joy--an emotion that is not conducive to perfect equanimity and stability.
The 3rd jhana, has left behind the flawed joy of the previous jhanas and is filled with extreme bliss and focus. This is nearly perfect but this bliss is so perfect and so amazing that it is said that the brahamas who are in the 3rd jhana heaven (the person died and was reborn in the 3rd jhana heaven) were totally physically blissed out... that they couldn't come down to listen to the Buddha's sermons, even at his invitation.
And finally, we come to the 4th jhana. The perfect place of one-pointedness. The place you want to be if you want to get things done so to speak because mental clarity and focus is at its apex and all the other (positive) slightly unstable qualities of the previous jhanas subside.
I hope this makes sense and I hope you will study the details for yourself further, especially by reading Bhikkhu Bodhi's explanations and The Mind Experiment by Bavo Lievens (from whom I shared the above story about 3rd jhana).
Initially you don't need much Jhana to start with. (This is needed towards the end of the journey but if you can develop it from the start it is well and good but not necessary. ) But wisdom is built on the foundations of concentration, so as your practice matures and wisdom increase you have to develop concentration to match wisdom.
In the general case, wisdom and concentration are co integrated, wisdom doesn't too far from the level of concentration you have, like a dog on a leash or 2 ends of a stick. (Maha Salayatanika Sutta mentions balancing the two.)
So it don't matter if wisdom or concentration develops in you 1st. If concentration develops use this as a tool to develop wisdom. If wisdom develops then use it to develop morality and concentration, and further use this to develop wisdom further. This is a cycle where you develop each item in increments.
So to start your insight practice shallow level of Jhana is sufficient but you have to deepen it to progress into higher insights.
Those intent on Anagami or Arhatship in this very life will require to gain mastery of the 4 jhanas. Sotapatti or Sakadagami does not require mastery of the jhanas, but requires a fair level of tranquil concentration akin to the first jhana. This level of concentration will materialize as a virtuous one with right view practices yoniso manasikara diligently. For Anagami and Arhatship even after the attainment of the two lower states one still requires to progress to the 4th jhana. We see the Buddha encouraging and advising Ven Maha Moggallana post Sotapatti with the latter's jhana practice. Your question reminded me of something I read about Ven Sariputta's teachings philosophy, where he was most concerned with a student up to the point the student attained to right view (ie. Sotapatti) thus assured of Nibbana albeit with several life times to go, verses Ven. Moggallana teaching philosophy where he considered any form of existence like excrement and encouraged his students to aim for nothing short of Arhatship, teaching them the mastery of the 4 Jhanas.