If I suspect something is up with the internet because Youtube or some streaming is sluggish, I test the speeds first to make sure I'm not throttled on my LTE Uncapped Telkom.
I only ever test when I notice something wrong - Speed test is mostly just another way to narrow down where the problem could be for me (i.e problem with my internet, or host internet)
I use it multiple times when I have to change the channel my wifi router is using because my neighbours have somehow migrated onto the channel I'm using causing interference and a borderline useless connection. I'm not even sure why this happens but speed tests are a great help in understanding when the issue is resolved.