Sharing one computer without stepping on each other's work
On a shared family or household computer, fast user switching means one person can hop in to check email or look something up on their own account while the other person's open documents, browser tabs, and unsaved work stay exactly as they left them in the background.
Multiple signed-in accounts do use more memory
Every account that stays signed in in the background continues consuming some memory, so on an older or lower-spec machine, leaving several accounts switched-in at once for a long time can make things feel sluggish. Fully signing out of accounts you are not actively using is worth doing occasionally.
Frequently Asked Questions
Does leaving multiple accounts switched in slow the computer down?
Each signed-in account uses additional memory in the background, so if you have several accounts and a lower-spec machine, you may notice some slowdown. Fully signing out unused accounts periodically helps.
Is there a keyboard shortcut for switching?
Yes -- press "Windows+L" to go to the lock screen, then choose a different account from the bottom-left corner of that screen to switch the same way.