When you download the engine from Epic (not the source code base from Github), you can still work in C++ if you want to, but it depends on what you’re planning to do with it. The source code is there for people who want to make changes to the underlying functionality of the engine. If instead, you want to do a project using the already existing features of the engine or building upon them, you don’t need the source code. You can even build plugins without needing the source code. If you want to mess with the underlying features, re-write some already existing functionality, make contributions to the engine, then get the source code.
If you have more questions about it, start a new question so more articulate people can explain it more clearly than I can 