Tapering body depth?

I am considering building a ukulele and I have some questions about body depth in acoustic instruments. I have noticed that on my dreadnought guitar and tenor uke the depth of the body (soundboard to back) gets progressively greater towards the base. Yet on my classical guitar and guitalele the body is a uniform depth.

What is the purpose of this change in body depth?

Why are some instruments built like this and others not?

Most, if not all professional builders will taper the sides. In my mind, any instrument that has straight sides, front to back, looks bad. The tapered sides are done for looks, comfort and better sound projection. It is as simple as that. If you are going to build string instruments, you should learn to do this IMO.

And by the way, most classical guitars are not the same thickness from front to back. The difference is not as great as a steel string guitar, but it is definitely tapered.

It's to get a different sound, although this is mainly done for steel-stringed instruments. For instance, the renaissance cittern had very tapered sides, and the technique is still used to a much lesser extent in some mandolins.