Great, thanks all for the info! Your four replies complemented each other pretty well, I think I have a grasp of the concept.
For the sake of giving an example of what made me to ask this question, I made two screenshots. I was working on a capture-the-flag style map and compiled it 1) without leaks and 2) with a small leak (I left a tiny unnoticeable spot open somewhere in the sky).
Without the leak, so the 'proper' version, some annoying spots of shadow popped up I can't seem to get rid of, and compiling times skyrocketed. With the leak, this wasn't the case so it made my life easier. Plus the maps looked almost identical - so I thought, no loss there, let's stick with the leak if it seems to have a positive effect.
But I wanted to know if there'd be a catch, and which mechanics are behind it, and you four explained that very well. I begin to see the consequences and limitations by not having VIS performed. Besides, some friends I play with have older laptops as well, so I'll aim for proper maps with this in mind.
Thanks again for the quick and clear explanation, cool!
First screenshot of the map, the 'proper' version without a leak:
Second screenshot, with leak, which looks almost identical: