Appcode folder is a folder that we can put our cs files (classes) in. For small projects you can use this folder to store your classes but if you put too many classes in that folder your building time increases
to improve the build time you should use class libraries.
(there are absolutely better reasons to use class libraries :) this is just one)
In object oriented principles every thing depends on classes. So we have to use many classes. It is good for maintenance but slow downs the performance.
Our systems should take care both of it.