So to get a bit more into details:
All of Marth's throws have weight-dependent throw release points, so you will have bigger frame advantages against light characters generally.
Up-throw has three main uses:
Up-throw is great to followup with a regrab (spacies only), up-tilt, and basically anything you can get from mid %s in.
That combo component is stronger against characters that are falling faster and are lighter basically. It works best between 20-40% (then also on average fallspeed), above and under that only if the criteria are fulfilled to a higer extent.
Up-throw is a kill move at really high %s (generally around 200%), you don't want to rely on it though.
Up-throw can lead to positional advantages if you fight a character having a bad landing. Your idea is basically to wait for them trying to land and then catch them with up-tilt/up-air. Works best against characters who have problems covering the space under them (Peach and many others to a lesser extent) or having bad horizontal mobility in the air (Snake, Lucario). Punishing landing is easier if platforms don't play a role or are away completely.
Forward-throw:
Use it as DI trap together with down-throw on light, floaty characters. Regrabs at low %, any move you can get (mainly fair and nair) at mid %s. Often you can also get an aerial followup or even a pivot/WD f-smash as kill setup.
At the ledge: Forward-throw-> Dash (really short) ->short hop -> dair works on a lot of characters, mainly those with less or average fallspeed.
Positioning: There are a few characters you can't really get any followups against, which are heavy, floaty characters (Samus, Charizard). Use the forward throw to bring them closer to the edge. Positioning instead of going for a combo which will miss can be really important, this kind of use is still underrated. The f- and d- throw positioning is better if they feel uncomfortable at the edge (bad recovery (?)), you can't combo on them (or at least not at theses percents), and they don't have a big problem with landing.
Down-throw:
Mainly the same as f-throw (DI trap), but with less followups. Use it when you DI trap and they expect the f-throw.
Positioning factor stays the same (if you are closer to the edge with the back, down-throw is your positioning tool).
Back-throw:
Really situational, similar to down-throw. Sometimes prefered in edgeguarding situations over the d-throw but generally your worst throw.
