I certainly wouldn't use it for gardens or flower beds. It was originally used for playgrounds, at least any tires WITHOUT steel belts. It's soft, doesn't decompose, and lasts forever, or nearly. For something under a swing set it's fine but I wouldn't use it any place else.
Any organic mulch actually pulls nutrients from the soil as it decomposes but eventually it all returns to the soil and helps it.
As the other poster said, they are trying to get rid of an unusable item, old tires. There are places for rubber mulch but not where you plant or grow things. Playgrounds, the area you store your garbage cans, maybe walkways, places like that.
Any organic mulch would work better and be better for the soil.