The grit guard argument has been raging for ever, they are either as important as breathing or as much use as a one legged man in an **** kicking contest, depending on who you ask.
So for my 2p, no they are not essential, good technique is more important. You can have a good wash technique and do without but you can also very easily use them with bad technique and get a bad result. Don’t want to turn this into the best wash technique debate but if you are taking a significant amount of dirt off the car with your wash media then it has to go somewhere, preferably not back on the car.
Do they stop grit from coming back up into the water? Yes. And no. The normal design (grit guard and Scratch Shield type) does trap larger particles, the cyclone design maybe smaller ones too, but the normal tiny bits that comprise most of the dirt on a car, no they don’t. The main problem is that in order to trap particles, those particles first have to sink to the bottom of the water, smaller particles either float around and never get trapped in the first place, or are simply sucked back up into the higher part of the bucket when the water is disturbed.
There was a video on’t tube that I can’t find that demonstrated the behaviour of particles in water when used with a grit guard. Although there was a difference, the grit guard did not keep all the particles trapped when the water was agitated. Stacking two gg on top of each other did make a dramatic difference, but it was still not 100% perfect.
So do I use them? Yes, partly because I already have them so why not, but mainly because when I empty out the buckets yes the water is dirty, meaning there is dirt in the water, but there is also dirt under the grit guard, which proves that they do catch some dirt, not all, but some, and every little helps.
So no, not essential, and they are definitely not as effective as the manufacturers would have you believe, but they are certainly not snake oil.