Conventions really help programmers that are unfamiliar with a system or project come up to speed much more quickly. Good conventions also help a new person understand common idioms, even those that are not strictly part of the code such as directory structures, where to find things on the network, how the organization uses version control, etc. etc. A good set of software development conventions are the genetic pool of a world class software organization. Things that don t work are discarded, things that work are improved. This isn t to say that conventions should be designed purely by committee, but that the synthesis of ideas from several people over a period of time is usually a good thing.
Conventions encourage and facilitate code evolution.
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.