One of the scenarios for use of the Glaser Safety Slug (and more in general, of frangible rounds) is urban combat and home defense in the USA.
Why in the USA? Because the prevalent way of building houses in the United States employs walls that are unable to stop most solid bullets.
A .45 handgun bullet (let alone a rifle bullet) will easily penetrate a drywall partition or a plywood door - this makes the use of solid rounds very dangerous; if you miss the hypothetical criminal, you may end up killing the invisible innocent bystander in the next apartment (or house, as the case may be).
birdshot ammunition for shotguns is also used for the same reasons; it will stop (and kill) a person at close range, but it is not very good at penetrating drywall, and it loses energy quite rapidly over distance (due to air resistance).