In an ideal world, wires would be ideal. They’d have no resistance, and therefore, no voltage drop. As we all know, our world is far from ideal which means so are our wires. While copper is an excellent conductor, it does have some resistance associated with it.
This means when designing DC circuits, very common in the hobby world, you may need to take into account the size of the wire you’re using.
David Norton at EDN has put together an excellent primer titled: What size and type of output wires should I use?. Give it a read, bookmark it, and if you use Evernote like me, clip it.