Giant tortoises live in the underground jungle and attack by tucking in and rolling at you down slopes, which is the entire fight. They hit hard enough for early hardmode that most people come back for shells after a mechanical boss instead of before. Nothing concentrates them either, so plan on a long sweep through the biome, or clear out a wide cave and let them wander to you while you fish or mine.

what they are for

They feed the turtle armor line, which is the heavy defense set of its tier and the step you have to pass through to reach the beetle set later. If you are not playing melee, there is no reason to touch a tortoise at all.

trivia

Nothing the game calls a turtle drops one. The actual turtles are critters and a mount, and the shells come off an animal named a tortoise.