Dear Misz Normie;
Slow release or retard drugs release according to their name the AS over a long period (mostly ~24h) but also with a relatively low and controlled concentration rate. The advantage is that you can keep the body concentration over such a long period nearly constant in the therapeutic range without taking a tablet at the morning, one at midday, and one at the evening. Also an advantage is that you never have a very high peak in body concentration, what some time can also be dangerous!
Contrary you need a relatively high body concentration in a very short time when an indication of a "pain" (analgesic) or for a sleep drug is given.
That’s maybe not all, but the most significant advantages for such indications.
Hoping to have been of help to you.