My husband thinks it is indeed for grip; that sand by itself just binds together too easily into a not-so-grippy mat. Sounds logical, but I don't know the official reason. One more reason why I hate winter roads!

Thank you weeper,.. he might be right.
A couple of boy's at the shop said they think it's because the extra expense for sifting out the rocks,.. in other words, sand and gravel is cheaper mix.

That's likely, too. I know it costs a lot more to get high quality gravel for our lane vs. a gravel/sand mix, so I imagine it is just the same for sand!