Delaying the Drawing of Trump
Getting some ruffs in the short hand before drawing trump is perhaps the most common reason why you want to delay drawing trump for a bit. I can recall the time when I had a spate of 5-4 trump holdings, all of which would have been a cakewalk on a 2-2 split, but where trump was splitting 3-1 time after time. Now in such a situation, you want to make allowance for the possibility of 3-1. If you have a lot of entries, you might get by cashing two top trump to test for 2-2 first, and then go about getting ruffs. But it's not so likely you'll have sufficient entries for that, and you'll want to prepare for the possibility. You can't take two rounds of trump if you must lose the lead to get your ruffs, for the defense might lead the third round themselves.
Basically, you wanna count your winners (or perhaps losers) and note how many ruffs you're gonna want in dummy (or maybe in the closed hand on a dummy reversal) and then set about assuring yourself of those ruffs. If quick re-entries are scarce to non-existent, you might, for instance, lose a trick in a 3-1 or 4-1 suit of no top cards, so as to have an entry back after a ruff. You may well be able to withstand a trump lead from the defense there, but not if you led trump twice and then saw how you'd have to lose that trick to get back to the closed hand, which may or may not mean a third round led by an opponent.
Below are some illustrations of where declarers butchered their contracts by drawing trump when they should've been getting ruffs (or at least one) in the short hand: