Why do I need a library file with -sources suffix?

Question

I don’t understand why libraries often have two files, one with a suffix -sources.

Here is what i mean

Answer 1

Sources are useful if you want to enter the library when debugging. You do not need them, but they can save you if you cannot understand why the library behaves in a certain way.

Answer 2

To add to the answer below, the source archive is the actual source code, and the other file is its compiled version.