If you can write all your classes in a single file in .net because you can declare classes unlike VB6, what are reasons you would have multiple class files in your application?

I can only thing of 2 and that is
1) if you have a ton of classes it can help to organize them (but so do code regions)
2) so you could use them in another project by just adding that .vb file that contains the class in it

if there are other reasons please let me know, just trying to figure out the best way to organize code going forward developing in .net