Grazing muzzles are designed to limit the amount of pasture grass that your horse can eat. Grazing muzzles also limit the size of the bite, so there’s less grass with every munch. Horses that are metabolically challenged, such as horses with insulin resistance or cushing’s disease, need limited pasture access, in this case the grazing muzzles are very useful and I think necessary. Using a grazing muzzle while your horse munches away on his most favorite meal ever seems like torture! But, if you don't use it, there will be many metabolic problems, possible laminitis, and all of the problems that come with having an overweight horse.
I would use a grazing muzzle if there was a lot of rich spring grass as this is the worst for laminitis and grass sickness. If you have a fat horse/pony then they are more susceptible to laminitis and you should consider a muzzle. I personally manage my horses grass intake with electric fencing and my pony who is susceptible to laminitis hasn't suffered from it since I bought him 14 years ago.