It is a good question that brings out an issue that needs to be emphasized.
1. First, ākāsānañcāyatana, viññāṇañcāyatana, ākiñcaññāyatana, and nevasaññānāsaññāyatana are NOT jhāna. They are samāpatti.
– The Buddha discussed only FOUR jhānās. These are the mental states of rupavacara Brahmas.
– One can proceed to higher arupavacara samāpatti once getting to the fourth jhana.
– However, these days, the higher four arupavacara samāpatti are incorrectly labeled as the fourth through the eighth jhāna.
– In many posts, I have also used that incorrect terminology. (Please let me know if you come across them so I can revise them).
In a recent post, I pointed out the above; see #4 of “Samādhi, Jhāna, and Sammā Samādhi”
– It was also discussed at the forum, “Post on “Samādhi, Jhāna, and Sammā Samādhi”
2. Now, to address the question of TripleGemStudent: “I’m wondering what are the benefits of those arupa jhana’s?”
The short answer is as follows:
– A cetovimutti (or ubhatovimutti) Arahant gets to nirodha samapatti by sequentially going through the first four jhana and then the four arupavacara samapatti. Thus, there are eight steps up to that stage. The ninth step is to advance to the next stage of nirodha samapatti.
– That nine-step process is also informative in the following way. That Arahant would transcend the kama loka once getting to the first jhana. Then they would advance through higher arupavacara states, reaching the nevasaññānāsaññāyatana at the eighth step, which is the highest arupavacara Brahma realm (dimension of neither perception nor non-perception). One is almost released from this world of 31 realms at that point.
– Even anariya yogis can reach that state. But they CAN NOT get to nirodha samapatti because they have NOT removed any anusaya! They will be born in that Brahma realm at death, but they can be reborn in apayas in future lives since they have not removed any anusaya.
– But A cetovimutti (or ubhatovimutti) Arahant can “temporarily detach” from this world of 31 realms for up to seven days. Since they still have kammic energy to sustain the physical body, they will be automatically released from nirodha samapatti after seven days (unless they willingly come off before that).