Class Libraries (DLL's) In .Net?

I'm creating an application that will utilize a lot of functions for performing scientific calculations. There will also be a lot of 3D and 2D rendering in my application. Hence i will have a lot of common functions that will be utilized frequently.

I have noticed that class libraries (DLL's) are utilized in applications similar to mine, and am wondering if i should utilize them in my application. The trouble is, i can't really think of what use they will be to me. The functions in my application will only be called upon by my application, so i do not need to make my functions accessible to others. The only other benefit i can see from using DLL's is reduced memory usage and .exe size.

Whats the benefit of using DLL's instead of just creating a bunch of modules in my application to store my functions? I have read that another benefit of using DLL's is they are easier to maintain and update as you can update the DLL rather than your entire application. I dont quite understand this, as if you are going to utilize new and updated functions you have added to your DLL's, then you will need to update the code that calls them as well. So again i don't see the benefit in using DLL's in my application.