One thing I would do being in a support environment is ask your senior engineer what was done. Never be afraid to ask!
To answer your question, it all depends on what is running on that server. If for example, it was running exchange there are exchange logs which can be removed which can take up a lot of memory (although exchange logs/datebase should be kept on a separate drive ;)).
WSUS (Windows server update services) is also something which can be cleared on servers. WSUS is designed to download windows updates to distribute to PC's on a network. Sometimes, if they are configured incorrectly they may download every update imaginable and store them. For example, if your client only has Windows 7 PC's on the network and the server has been set up to download Windows XP-8.1, the other operating system updates can be removed as they are not needed.
But as I said, it all depends on what the server is for and what is stored on there!
It's very hard to imagine all the crazy things that things really are like.
Electrons act like waves.. no they don't exactly, they act like particles.. no they don't exactly.
Words and ideas can change the world.