We've often heard the adage 'The right tool for the right job' in probably every possible field and also perhaps its contrary counterpart, 'An axe is a clumsy tool to open a can of milk'.  I've often learned the value of these statements first-hand, while working on my bicycles.  All too often, a tool, even a tiny one, does work which simply cannot be achieved without it, without going through a whole lot of difficulty.  Two actual tools from the world of bicycle maintenance that come to the mind straight away are the chain-link removal tool and the crank puller.  These tools make it extremely trivial to accomplish the tasks that they are designed for and without them,  there is no end to frustration and wasted effort.

The chain-link tool: can be used to break or join links from a bicycle chain.  It's incredibly hard to do it without the tool and to ensure that the chain remains free of kinks and/or stiff links.

The crank puller:  without this,  the crank arm needs to be hammered out of place which could potentially damage it.  On the crankset side, it's worse, as the gear teeth might get bent or even break, when a hammer or crowbar is applied.