1.Fighting Aizen in that evolved hougyoku form needs crazy amounts of reisatsu which even a hollowfied Ichigo didn't possess.So to match aizens reiatsu he trained for the FGT.and here are the two other questions then.. because I start to think there's a plot hole
Q . 1 why... why did Ichigo use Final blablabla if he was without it capable of fighting Aizen
Q . 2 how was Ichigo and why part of what Aizens plan? so much effort and all he wanted is just to kill him... what's the point?
2.Aizen's plan was to overthrow the soulking and to take over the throne.He knew about Ichigo's mom being a quincy and his dad being a shinigami.He expected Ichigo to be special and provided ideal conditions for his development.So basically he used him as a test subject.When Ichigo turned out to be more powerful in their last fight,Aizen went nuts and tried to kill Ichigo.I think the soul king is a Transcendental being with all the powers i.e, shinigami,hollow,quincy much like Ichigo.So he wanted to test the possibility on Ichigo who was an ideal choice at that time.