The fuse protects the conductors (keeps them from going up in smoke and burring your house down). Typically, home wiring is done with either 14 and 12 gauge. NEC tables only allow 20 amps for 12 gauge THNN. 14 gauge is good for 15 amp. I am only speculating, but I bet the other fuses you mention that are 25 / 30, are there because the circuits kept tripping and the last person just screwed a larger fuse in.