You don't really need one if you have any form of mechanical sympathy and a brain between your ears. I've got 2 (I like tools, OK!) that I've bought for other stuff and the only thing I regularly use them for on a bike is doing up casette lockrings. Cassettes with alloy carriers/lockrings like SRAM PG990s can deform if you do them up too tight.
But otherwise, I've never had a problem
You'd be better spending a few quid on a tin of copper grease and dabbing a bit on all your bolt threads before you tighten them up, especially disc rotor bolts. That will hepl in the long run, believe me!