What's the hold function on a multimeter for?
I rarely use the feature but for more precise measurements it can be useful. For example if measuring a sensor I might want to make a note that the meter reads 2.984 V and it's more convenient than trying to remember the value while I write it down on a piece of paper or enter the value into a spreadsheet etc. 

Sometimes too I might be in a position where it's hard to read the meter output at the same time and I can rest my little finger on the hold while positioning the probes.

Some higher end meters also have an auto-hold type feature where once the reading becomes stable they will take the measurement and hold the display. That's a feature that I find much more useful because you don't have to keep your eye on the meter at all and they will beep when the measurement is done.