Question

Why is it generally a good idea to set the font size to at least 30 points in presentations

Answer 1

Just try it. Make a presentation with font-size 10 and look at it from a distance (like 10 metres).

Answer 2

Looking at the font from the back row of a presentation, anything smaller than 25 is very hard to read.

Also, if you need lots of words to describe your presentation, you haven't simplified it enough for your audience.