Assuming you go to the same garage, and that the mechs are not the owners... Your car is one of many at the garage. You may have particular expectations on time or how you like your settings (specific wheel alignment, etc) A bit of extra "appreciation" means your car may get fixed earlier, and come back with a few extra things adjusted without being on the bill, along with extra care to meet your specific asks.

Or so I was told. I saw it more when I was younger than I do today.

And somehow I get that in the rest of the world by just being a regular customer, without paying a tip.