In your case (older MBP with an Intel processor running macOS Big Sur), you cannot upgrade to Fusion 13 at present, because Fusion 13 won't run on macOS Big Sur.
Assuming your Mac isn't too old, then at some point you may want to upgrade it to a newer macOS version, e.g. because macOS Big Sur will stop getting security updates in late 2023.
If you upgraded to macOS Monterey you could run Fusion 12 or 13: in that case the main reason to get 13 would be if you needed to run a newer guest operating system which wasn't supported by Fusion 12, or you wanted new features of Fusion 13.
If you upgraded to macOS Ventura you would have to upgrade to Fusion 13, because Fusion 12 doesn't support Ventura.
See the release notes for Fusion 13 to get an idea about new features:
https://docs.vmware.com/en/VMware-Fusion/13.0/rn/vmware-fusion-130-release-notes/index.html
MBP models and supported macOS versions (only considering Intel models able to run Big Sur):
- Late 2013 and Mid 2014: Big Sur is the last supported macOS version. You would need to replace the Mac with a newer model to run macOS Monterey or later and keep using Fusion. (Note that if you need to run x86-based operating systems in virtual machines, your new Mac must also have an Intel processor, which limits to you considering 2015 through 2020 models, not Apple Silicon models with M1 or later.)
- Early 2015 and 2016: Monterey is the last supported macOS version.
- 2017 through 2020: can run Monterey or Ventura, future macOS support not yet known.
