Frogs come in all sorts of descriptions these days and like the rest of the horses foot are completely dependant upon either the importance given to them in the breeding process of it's last few hundred generations, or just sheer luck. Breeding programs have not been kind to the horses frog. The lucky few who have been left with healthy natural frog conformation have a nice wide tough frog that grows robustly with shallow commissures on each side. On the other end of the spectrum is the 'almost no frog' which refuses to grow much. Or a tall frog that tips over when asked to bear weight and is just a nice habitat for bacteria and rot.
My only reminder here is that frogs shed naturally-they just come loose and fall off-several times a year. You can see a video of that here. Many times when this happens they will dangle around for weeks and when your farrier shows up he will have to cut it off. If the horse were traveling and browsing for 20 miles a day as they were once designed to then the shed frog would quickly find its way off the foot. When they don't they can sometimes cause sores on the sole of the foot.
Sometimes the normal frog will be infested with rot, from the hooves not being picked out once in a while, which will then eat its way into the foot and eventually get to sensitive material, causing the horse pain, and to limp. This is called thrush. These rotten frogs also need to be aired out and cleaned up by the farrier.
But frogs are meant to be left intact to serve the important function of absorbing and cushioning impact, as well as helping to pump blood the long distance back to the heart.
So ideally the frog needs to be left fully alone by the farrier and owner unless these several other factors come into play.
And the central part of the heel, the cleft of the frog(central culcus), is the most popular place for rot to start and needs to be picked out occasionally as well. Some horses don't have much of one while others have cleft's which are very deep. The deep ones, staying moist and airless are the lush breading grounds for rot causing bacteria, leading to painful lameness for the horse. Sweep it out with your hoof pick and occasionally pour some Clorox in there while holding the foot up and letting it soak down in. For horses with painful sores already that cause them to kick out at you when cleaned, you'll use something less harsh like Thrush Buster. Among other things Thrush buster has in it iodine, Clorox, and a drying agent which will leach out that all important moisture and kill the bacteria in only one or two applications.