It really depends on the exact model of the Mac. On a 2018+ Mac with T2 security chip or an M1/M2 Mac with Apple Silicon, enabling Filevault just adds an extra password to protect the system since the data on the internal SSD is already hardware encrypted -- the process is finished in seconds.
With an older Mac, Filevault will encrypt the entire drive so if the Mac is lost or stolen, then whoever has the Mac won't be able to access your files on the drive, although the protection is only as good as the password used for each user account on the Mac. If Filevault was not enabled on the older Mac without the T2 security chip, then anyone who has physical access to that Mac could connect the Mac to another computer (or possibly remove the drive on some models) and get immediate access to the data on the drive -- this is not possible if Filevault is enabled (again, it is only as secure as the passwords used for the macOS user accounts on that Mac).
Here are two Apple articles regarding Filevault on both the older Macs and on the 2018+ Macs:
Use FileVault to encrypt your Mac startup disk - Apple Support
How does FileVault encryption work on a Mac? - Apple Support
2018+ Macs:
Encrypt Mac data with FileVault - Apple Support
Personally I would enable and use Filevault since it is so easy to use. Even on older Macs with slow spinning hard drives, it never really impacted performance in any significant way once the initial encryption process finished.
The only con is if you don't have good backups and you forget your password, and you lose the recovery key, then you would lose access to the encrypted data since the only way to get the Mac working again would be to perform a clean install of macOS by first erasing the encrypted drive before restoring from a backup.
People should always have frequent and regular backups of their computer and all external media (including the cloud) which contains important & unique data. FYI, There are a lot of ways to lose your data these days especially with the USB-C Macs and even more so with the 2018+ Macs. Some involve accidents, some involve system/software bugs, others due to hardware failure.
So is there really a con? I don't think so.
Just curious if this is your own personal computer and purchased new from Apple or an authorized reseller? If this was a used Mac, or one which you have your school or employer manage, then maybe the Mac is being managed and configured by the school or business to force Filevault to be on. I have seen a few other thread on this forum where people discovered their Mac was still being managed by a previous owner (or by their school or employer) after upgrading to Ventura.
