Thanks for enjoying this silly game :D
The extra heart is the reward for doing all 3 quests Biboo gives you. Not essential to winning (especially if you found the IRyS fairy) but a fun gift for good pebbles.
The SNES cartridge reason? That's the console I grew up on as a toddler.
It was a tough balance to have a time limit for a world with exploration. I used the Biboo quests to guide you to what I consider the essential items, but left the rest to be optional. Curious players could explore. Busy players can go straight to the boss. It's totally fine to not get everything.
Wait, did you miss the hammer too??? :D