Note that since FreeBSD 13, FreeBSD has been rebased and now uses ZFS on Linux rather than its own, separately developed and maintained ZFS stack. FreeBSD 13.1 upgrades to a newer release of ZoL, which is very important to note since the version that shipped with FreeBSD 13 had some notorious corruption bugs surrounding some of the newer ZoL features when sending/receiving snapshots with unmatched ZFS dataset record sizes or with native ZFS encryption (previously altogether unavailable for FreeBSD ZFS users) in use.

ZoL is still ironing out the remaining corruption bugs in these features, but the snapshot in FreeBSD 13.1 is a much more reliable option than the one that shipped with v13.

Note that users are not locked into the version that FreeBSD shipped with; you can actually installing rolling ZoL releases via ports/pkg and even use them for the system volume but that requires some reconfiguration (installing the port/pkg plus a minor conf file change to load the desired version of the ZFS kernel module) - but since most users don't do that, this should be a welcome upgrade.