long post, most of this is already known, but if this is going to be a guide, it's good to reiterate.
I love doing downward db3 to strutterstep fsmash if I've connected w/ DB/they're close on the ledge and can't afford to get the punish oos as easily (usually if they're conditioned to expect just the regular full sideB), dtilt (really safe), ftilt/jab (if they jump/try to go upwards), or neutral B (long range, sometimes for shielding people) at really low percents if I am more familiar with how they would react out of it, many times I get a tipper right off the bat, which is especially nice w/ fsmash for getting a good chunk of damage. Higher percents, you just shield or jump and see what the opponent does, and you usually get some good hits in regardless.
As usual, DB1 has some good use outside of its traditional use while in the air. It helps w/ recovery a little by getting that boost, mixing up your recovery, or slowing your fall (at the cost of horizontal movement iirc), which can mean the difference between your upB getting stopped by invincibility and it knocking them just when they are all out. It's also a decent preemptive mixup against tornado in the MK MU while wavebouncing it in the air. Sadly, DB1 as a gimmicky stringing move is not so useful as it used to be as more people are familiar with it, but still works sometimes for landing strings.
upB's entire upward variation is great for certain characters that have good recoveries that aren't worth doing much for edgeguarding, but are prime for juggling (snake and ROB come to mind, although both are still fairly exploitable in edgeguarding).
Extremely gimmicky, but something I've noticed: Funny thing when you've got a pocket marth, everyone recognizes the "novice marth"/stressed or choked failed upB (the accidental, super rushed db1 + upward 2nd hit, then a panicked shield/upB after), I've gotten a couple of good opportunities from the opponent expecting something like upB or shield, when in fact you can just retreat nair or whatnot and get away, or even grab them or something. Really situational and probably not too useful at a higher level of play, but don't be surprised when the opponent shields or spotdodges that variation after you do it expecting an upB, especially oos. Ignore this, falls under mindgames more than mixups and isn't steadily reliable for something like this lol
DB1 -> grab still works sometimes, it's more useful for landing the grab while chasing down after a throw or something to abuse that minute landing lag and create the trap w/o them having as many options to escape as when they are on the ground. Also works pretty well if you've gotten a successful crossup, because unless they have a good upB oos, they'll often act really predictably to that poke because they can't just grab you when you're behind them.
clowsui is right, a lot of people don't expect any more of a full DB string coming down from the air, they often drop shield for some odd reason. It's kinda something I played around with, you definitely need to do it at a certain height or it's really obvious as you're coming down.
DB's general purposes are still very undervalued by some imo, just because even getting a full DB at low percents immediately does several things:
-refreshes your moveset
-covers options like airdodge and spotdodge if it hasn't hit them yet
-because the initial knockback is actually fairly strong on the last hit, it puts them offstage a lot. Opponent being offstage = good for Marth period, even if it means only securing control of the stage and putting your opponent in a bad spot.
