I just got a used savage and the rear diff is locked. What is the point of this?

Probably for the same reason that full scale trucks are avalible with locking rear differentials. Basically, so it's less likely that you'll be in a situation where you'll be hopelessly stuck, because the Savage and full scale 4WD trucks only have diffs going side to side, but not front to back (well, unless you have a transfer case with an interaxle diff, but that's another story). So, if you get opposite corner wheels or both left or both right wheels stuck, you're stuck. I see it as no real advantage on an RC though, since you can't turn it on and off, and I don't know about you, but very seldom do I drive on terrain loose enough where a locked rear diff would be beneficial.

the diff housing has a grub screw which is visible and if you loosen it a little at a time it will revert it back to normal.
I discovered this when I saw the grub screw and thought it was loose, so i tightened it and the diff locked. I then unscrewed it a little and all returned to normal.

Ive locked my front and rear ....the point is ,you dont have to replace diff gears near as often, if at all ....ever again.
With a hi torque servo turning is not affected.