A friend is a cook who says that even at decent restaurants, cooks don't get paid much and they get just a minimal share of tips. So when I get takeout, I tip 10-20% and say "Thank-you, here's something for the kitchen." Sometimes I get thanks in return, sometimes just a sour look from a server who expected something more for putting my order in a bag. I have no idea what they'll do with the cash, but hopefully some of it gets to the back of the house.
The cash aspect was incidental. "Front of the house" workers here net a significantly higher take-home because they are the ones receiving the tips and what they pass on to others is typically discretionary. Even many of the best establishments will pay cooks subsistence wages. A few restaurants are going tipless, raising prices slightly and discouraging tips, so they can distribute pay more equitably, but then they can have problems retaining wait staff.
This was my understanding as well. If they just ring up the total and pass you food, there's no expectation of a tip.
I do tip from time to time at places where I'm a regular customer. If I get takeout somewhere near work of home ~4 times a month I'll drop a dollar in once every couple of trips.
It's not normal to tip for takeout, but if you want to, 10% is the standard tip as opposed to 15% for table service. But again, a takeout tip is not expected, especially for a simple order.