If you can correctly add all the required explicit bounds checks in C what do you need Fil-C for?

Same reason any turing complete language needs any constructs - to help the programmer and identify/block "unsafe" constructs.

Programming languages have always been more about what they don't let you do rather than what they do - and where that lies on the spectrum of blocking "Possibly Valid" constructs vs "Possibly Invalid".

For temporal memory safety.