In some circumstances, you may need to obtain a quiver or a bomb bag early, for exemple if you performed Early Master Sword and and got the Light Arrows first, without ever entering Tower of the Gods, or if you got the Bombs in the Earth Temple Trial without ever obtaining the bombs in the pirate ship. Light Arrows and Fire & Ice Arrows, give you a bow but not a quiver, so they won't be usable before obtaining one. The Bomb drop in the trial gives you bombs but no bomb bag. That is where entering Fairy Fountains early can become useful.
If you have chest storage, you can clip inside fairy islands by walking very slowly in the back of the island.
Note: Getting a quiver or bomb bag early does not give you the bow or bombs as usable items.
Same as with chest storage, if you have door cancel you can phase through the walls of the fairy fountains to enter the loading zone.
If you enter Thorned Fairy Island early specifically for an early quiver, the only thing preventing you from exiting normally (without savewarping/deathwarping) are vine tentacles. It is possible to get around them in 2 ways:
Sidle on the left side, then go right to sidle against the barricade. Continue sidling until you hit the right corner, then squeeze yourself through by first crawling then rolling.