What are the pros using a build server with sharepoint when having projects your working alone at?
I'd say the biggest is that you greatly reduce the potential for human error. Say there are ten steps that you take for a deployment. If it's a person doing them, then it's likely they might forget one or do one incorrectly. With an automated process you can be sure that it's doing the same thing every time.
In addition to the advantages of automation, having a separate build server verifies that everything you need is in source control. Development environments tend to end up with forgotten dependencies. You need to be able to build from source control in a clean environment.
Of course, all of these considerationss apply to software development generally, not specifically SharePoint.