Aqua It's because it would be awesome to have access to best-in-slot (BiS) gear with a lot less RNG and gamble. We already have access to upgrade the enchant levels to max for Constellation Tower set, the former BiS, and the 3rd and 4th slot of Varmundt Biosphere rings, the current BiS accessories; and said gear do not have an upgrade option on vanilla servers, yet we got it. So why not expand that option to Temporal Circlet? It would also give endgame supports an option to expand their gear (Adamantine / Affection)
Temporal Circlets (TC) are also limited to the job you get it for. The MVP headgears are universal to that category of job.
(Vesper = Magic / Boitata = Crit / Beelz = Non-crit). Considering grading an item to Grade A costs around 200-400m at average, that's already good for universal lategame gear. For TC, the current method of maxing it can single-handedly cost you billions unless you have insane luck; and you can only use it on a single job.
So being able to max Temporal Circlets are neat considering the budget you need, and helps with late-game gearing.