Source Vs UE4 Installer?

I hope this is in the right place if it is not I apologise profusely. I question is probably a simple one but for the life of me I cant find the answer so either I’m looking in the wrong place or as is most likely I’ve been searching for the wrong thing.

What is the difference in the source that can be downloaded from and then the Installer version. So far whilst i have downloaded the source I’ve been using the Installer version to create my projects. Is there really a difference between the two and which of them do you all suggest using and why?

I thank you all in advance for you time.

Thanks for the reply, it makes more sense now.