When another's attack would do more damage/be more impactful than yours.

Not going to lie: I don't feel like going through your party and doing the math, but suffice it to say that there are times--even when playing a Great Weapon Master--that someone else's attack is expected to do more damage. Like when you can direct your Sharpshooting ranger to take a shot at the dragon overhead that you can't reach. Or when your L15 rogue is doing 8d6 or so on a hit.

I recently played a Battlemaster in a L18/19 party--being able to forego 1 of 3 attacks to give the rogue a sneak attack for 10d6 was pretty nice. (Especially when my first attack had been a Distracting one to ~double the rogue's chance of critting and doubling those dice!)

At low level, though, you're right: your attacks are generally going to have more "OOMPH" to them; delegating one to another party member is likely to be a strategic move: interrupt a caster's concentration when you can't reach them, an ally has a time-limited buff, you have disadvantage from being blinded, &c.

TL;DR: Commander's Strike gives you, as an option to replace any one of your attacks with any one of your allies' attacks. Options=power. The cost of that option is a bonus action and the opportunity cost of choosing Commander's Strike over another maneuver. I've generally found it worthwhile as a 4th or 5th maneuver pickup.

Make your worldbuilding believable

You've told us that this person has thousands of legendary items. That's enough wealth to buy the kingdom. That's enough wealth to buy all the kingdoms!

If he can make legendary items that cheaply, then why is he a bandit, instead of the greatest artificer ever known?

Is the rest of his gear similarly legendary? Is he wearing +3 armor and a +3 shield, et cetera, et cetera?

The books that give +2 stat are only "very rare". Has he manufactured ten of each book and maxed out all his stats? Has he done the same for all his friends?

I don't think there's a believable way to give this guy that much wealth and still have him be a bandit king.

The problem is the wealth is being used inefficiently

It's like if you did a fight against the US President, and he had an attack called the Billion Dollar Bill Chaingun, which was a gun that shot rolled-up billion-dollar bills at people.

Players would say: "okay, we get it, this guy is absurdly insanely rich. But is this really the best use he could think of for that money?"

Your proposed trick requires house rules to work

You've told us that the bandit would "turn the bag inside out at someone", but there are no rules for how many talismans would touch the target if this happened. The rules just say:

If the bag is turned inside out, its contents spill forth, unharmed...

and it's not clear that the contents would touch anyone, and it's also not clear that more of them would touch the player character than the person who's actually holding the bag.

You can houserule this to work, of course. But if you're willing to use DM power to produce house rules for this, it seems like you could houserule something that would be simpler and more believable, and it would make for a better game.