My mum was a waitress in the 30's.
It was a part-time job as outside restaurant hours she was "in service" to the owners (a B&B with a small restaurant). She was pregnant when she was sent there and worked virtually until my brother's birth.
She taught me to tip as tipping was her only income. Her job was as a house maid so she was up at 5 setting fires, heating water etc. for guests and owners. She did the laundry and cleaning before going on shift as a waitress. She'd be lucky to be in bed by 11 most nights and was relieved to be taken on by a munitions factory when the War came (despiite the dangers). She wasn't paid as she was given "free" board and meals!
Her tips would pay for her weekend home once a month and daily living needs.
Times have changed, though. Minimum wage means that people no longer live off tips like my mum did, but tipping is part of our culture so I still do.
I dislike service charges and always tip in cash - I tip for the service from the person who waited my table. I also always make up the tip for friends who refuse to. I've given up asking for their share even though they earn more than me.
My experience of wait staff has been wide - I was a barmaid and waitress while at uni. I'm chatty and tend to ask how they've come to their jobs. A lot are students, some are lifers that started as waitresses and have actually enjoyed their jobs. Increasingly I'm seeing older women waiting as a living after divorce. I've even come across a young woman who worked in the nursing profession who waited part-time to pay for a divorce her STBX wasn't expecting.
My point is that the person bringing you your dinner tonight might be writing your prescription in a few years so don't judge. My mum went from humble beginnings to owning a successful business - but never forgot to tip.