"Spam" is unwanted, unsolicited commercial email, and in massive quantities it is annoying and potentially risky because it can include bogus offers that will cost you time and/or money.
In Office 365, your spam messages are placed in quarantine. You have the ability to review and release messages from quarantine into your inbox.
Clutter is a mailbox management feature available to you that can be turned on or off.
Clutter differs from "junk" email because it comes from valid sources or people that you have interacted with before. The message is not unsolicited (like junk), but it may be low priority to you. So the Clutter folder is basically your second-tier Inbox.
If you receive a lot of emails a day, the Clutter function might be a real time-saver long-term, since Clutter gets smarter as you use it.