You serve me, but the person you serve me to doesn’t eat me. What am I?

What do you drop when you need it, and pick up when you don’t?

A tale of two deserts vying for sand, One desert must fall when united they stand, One desert diminished, the other replete, When the world turns, this tale will repeat.

Measure me or scan me then add me up. That will tell you how many there is for you to spare at dinner time. Maybe skip the dessert. Too many of me in there

Turn on it Spend it Drop it Stop on it The answer is worth more than a thin one