Grappler build

[HiveStriker]

Since the Grappler Feat does exactly this:

You have advantage on attack rolls against a creature you are grappling.
You can use your action to try to pin a creature grappled by you. To do so, make another grapple check. If you succeed, you and the creature are both restrained until the grapple ends.

It's mostly only useful if you want to lock down movement without providing disadvantage to your Ranged allies (at the cost of locking yourself down too).

I can think of some other ways I'd like to spend an ASI for a grappler.

*Sigh* How many times will I get to read this?

The important part is here: "You have advantage on attack rolls against a creature you are grappling."
The pinning part is the actual accessory.

On a Barbarian, Grappler feat is probably redundant most of the time: you can get advantage with Reckless Attack if need be, or use some archetype feature (at least one Totem has it) to get a creature prone as part of an attack or move. It would definitely be worth on an Elk / Bear / Eagle Totem Barbarian though, forgot that particular build.

On a Fighter, past level 11, it would probably become equally situational, because even if you want to Shove prone since you now get 3 attacks per Attack you can afford to "consume" two attacks.
Plus, unless being Eldricht Knight, you'll have too reduced of a speed to make a regular use of the dragging ability, so you'll probably prefer just Shoving enemy, Grapple it then smack him while being confident on your ability to survive without moving much.

On a Monk? You could use your bonus action on another attack because you want as many chances to Stun as possible, or you could want to use it on something else entirely, like Dodging to facetank the grappled creature. If in addition to that, you want to Shove, then you deal 0 damage on the first turn. Not very important for a target that has enough HP to "last" several rounds, but for lesser creatures it's a regular waste of attack potential. Especially when you want to Dash.
And there should be many reasons to Dash while grappling creatures unless you really have nobody with any lasting AOE.
Furthermore, keeping the creature "standing" indeed does not hinder your ranged attackers, but it can also provide a +2 AC against ranged attacks from its friends (of course, it may also mean you are making cover for it for your friends depending on relative positioning and things, but that's better than cover AND disadvantage nonetheless).

On a Rogue? It's the right way to be completely self-sufficient in enabling Sneak Attack (which, let's recall, requires advantage or melee ally) without having to rely on dual-wielding, which would be troubling without Unarmed Strike or additional limb since otherwise you do have one hand taken by the grapple (and unless I'm mistaken, sadly, by RAW and probably RAI using your Attack's weapon attack on a Grapple would not qualify as enabling two-weapon fighting weapon attack). Meaning you can safely land Booming Blade + Sneak Attack with high chance of success with your action, then Dash as bonus action to put yourself and/or the creature wherever you see best fit.
Besides, considering the amount of damage a level 10 AT would land (Booming Blade + rapier + Sneak Attack = 1d8+5+1d8+5d6, average 31.5) as long as party focus fire a bit you should be able to kill a creature quickly. Meaning opportunity to start another Grapple in the same fight.

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Anyways...
Moving creatures around is *extremely* powerful unless you're really alone and without any environmental hazard that you could use. The main problem is that considering the movement spent to get to creature, and the movement to spend to drag optimally, you want at least a "reserve" of 20-25 feet to have viable tactical options. So this kind of mindset requires specific classes, features, or spells (from you or friend). And Grappler enhance your attacks while letting you focus on that, so the more damage you can deal on single hit and/or the less number of attacks you get the better it is.

In long fights where you grapple the same creature for many and many rounds, Grappler would definitely be worthless for most people (Rogue still apart), since you'd "waste" only one extra weapon attack on the first round, then have "full attack" all the other ones...
Thing is, many people around here tend to say that fights are short, with an average of 3-4 rounds. So every bit of spared offense is proportionally much more important.

With that said, the real value of Grappler, as in all feats, will depend on many things: own character, party composition, and ability to teamwork. For example, if you have a Barbarian and a melee Battlemaster, chances are you'll manage most of the time to coordinate to Shove the creature and gang upon it, so then whatever class you are Grappler would rarely come into play.

Opposite example: 4E Monk with a Druid liking Spike Growth as his mundane tactic can deal huge amount of damage for little effort once Grapple is on.