What is the difference between APFS Local Snapshots and Time Machine Snapshots?
Both are backup utilities and both offer you the ability to recover after a &quot;catastrophic&quot; event, but like tools in your tool box, your box wrench and your socket wrench both essentially have the same function, but have their preferred application depending on the scenario.

APFS snapshots are a point-in-time snapshot of your APFS file system.  Your initial TM backup may be a point in time snapshot, but subsequently, it's an ongoing differential backup - only backing up what has changed.

APFS snapshots are only done on drives that have the APFS file system. Time Machine will back up any supported drive, APFS or not.

APFS are not so automatic.  Yes, they are done automatically prior to an update being applied and you can script them using launchd or cron, but they're generally something that gets done only before a major system change.  Time Machine on the other hand, by its very nature not only runs on schedule, but runs continuously to ensure data protection.

APFS snapshots are stored to the local APFS drive.  This means that you need to have ample space to take a snapshot.  TM doesn't care how full your drive is to take a backup as long as you have a target drive big enough to back it up.

Since APFS snapshots are only stored on disks with the APFS filesystem, when you run out of space, earlier snapshots are automatically deleted.  While this is also the behavior for TM, you can easily add to your backup space by swapping out another external back up drive.


TL;DR
APFS snapshots should not be used as a method of backup because they are only point-in-time snapshots of your system.  This is useful if you are about to do an update or make a change and things go sideways and you need the ability to return to a known working state.
Time Machine offers true back up capabilities because it offers more than a point-in-time backup, but an ongoing differential backup where it gives you granular control of what you back up and restore regardless of filesystem.