You need Hieda Scrolls to use reincarnation. You get your first by fusing all those pages you got for beating the dungeons on the moon. Beating the Emergency! dungeon is the first time you can a scroll. You can later get more scrolls as rewards for clearing the new dungeons you unlock.
Most important seal in the game is Cure, which restores your HP when you deal damage. You can get it from the Feather Duster Thingy weapon. Combine that with the Vitality Enchancement skill that the Goopy Banana Bunch armor has which increases the healing rate of everything and you will heal so much HP with every attack.
There is also stuff like Hisou, Multi Level Attack, One-Hit Kill, Attack Range up, Penetrate Attack, Defense Ignore. Really, there are a ton of good seals to have. Becomes more important once you can reincarnate into weapons and carry over their yellow seals though.