iTunes backup files are big. It could eat your storage space pretty quickly, especially when you have an iPhone with thousands of pictures and videos captured over the years. What’s worse, it’s always set up in your profile that can be synced to the server if roaming profile is enabled.
Normally, if you are running the traditional desktop version of iTunes, you can go to the following folder and manually delete everything in there to clean up the backup files.
%appdata%\Apple Computer\MobileSync\Backup

But if you are using the iTunes app from
%userprofile%\Apple\MobileSync\Backup

Luckily, you can also go to Edit > Preferences in iTunes app, switch over to Devices tab, and clean up the backups there too, which is much easier because you also get to see which copy of the backup file you are going to delete.

To further free up more spaces, you can go to the following folder to clean up the downloaded software updates as well.
%LocalAppData%\Packages\AppleInc.iTunes_nzyj5cx40ttqa\LocalCache\Roaming\Apple Computer\iTunes\iPhone Software Updates