Evolution is not really code reuse, although this is an important component, but more like building a pyramid, layer on layer, earlier work supports later work. Conventions help programmers from repeating bad history. While its obviously important to write good code. Its just as important not to write bad code. Good conventions can help document this or make it more clear.
Conventions can do great things to encourage and facilitate portability.
This is really important when working with multiple operating systems or target processors. Conventions help the programmer really think about what he or she is doing. Unless you are lucky, a quick hack will usually come back to haunt you. Sticking to conventions help you do things right the first time, instead of hacking it the first time, and doing it right the second time. Or worse, patching a hack.