I will never understand tipping a barista. You are tipping someone for handing you a drink.
From one of my Men's Health mags years ago....their advice when tipping a wait person/bartender, etc. was to
not wait until the end of the night to leave a big tip.
Give it to them on the first drink. That way you'll get the best service - maybe including stiffer drinks - the entire evening.
I thought that advice made a lot of sense.
To answer your question - because it's usually in
your best interest to do so......like making an investment.....
BTW....I stand by my recommendation to you to get a subscription to that mag.....for single guys it's a blessing.......