Hardware can break, making it impossible to get your data off your computer.
Software can fail, corrupting or even deleting your important files.
You may even accidentally delete an important file.
Data can also be lost as a result of computer theft.
Many files are irreplaceable. Backups protect against loss of files. Only the very latest changes would be lost, depending on frequency of backups.
Additionally, it may be good to have access to older versions of certain files.