Australia

You're correct that restrictions only effect law-abiding people.

I think there's a few things going on.

Firstly, every gun has the potential to cause harm, so less guns is less potential. Halving the number of guns does not halve the risk of harm, but there's some reduction.

Secondly, I value living in a country where there just aren't that many guns around. We don't really have any kind of "gun culture" here. Sure some people feel the need to have a firearm to feel safe but they're a small minority.

Also, and this is related to both points, without buy backs the number of guns in the system will always increase, as old weapons are more likely to be stored rather than destroyed.

Finally, it reduces the opposition to the new laws. Suppose a decade ago you purchased a weapon that you use for legitimate vermin control but is now illegal. You're not particularly concerned about that but you still need a weapon for that purpose. At least this way you're given a chunk of the cost of a replacement.