skribs
Grandmaster
I didn't say holding, I said slow motion under self imposed tension during extension. So, you are working on extension (and also on retarding that extension, which in the case of a side kick works on the bits you use for a hook kick).
You still need to lift your leg up to the side to do a fast side kick too. Doing it slowly helps increase height and accuracy for your fast ones.
But when you do it slower, you use less force for the extension and more force to hold your leg up, even if it's not as much of a difference as holding your leg out.
It can be a useful practice, but it's not building more muscle in that direction.