Fast-forward in checkout is quite useful, when you are moving for instance another branch to current commit.

Benefits:

• with huge number of changes, the operation is still fast (you need not to do two checkouts: one back to the other branch and one caused by after fast-forward)
• without this feature, in case you have opened an IDE (for example Visual Studio), it may rebuild intellisense two times – which could be slow more complex projects

https://answers.atlassian.com/questions/10395574/