Does anyone use these on a regular basis or are these used primarily by gunsmiths?

I saw these on Bushmaster's web site with the following description.

GO-GAUGE - used to determine adequate headspace. Chamber depth enough or oversized bolt.

NO-GO GAUGE - used to determine excess headspace. Chamber depth too much or undersized bolt.

FIELD GAUGE - used to determine overly excessive headspace due to chamber and bolt wear.

Although they provide a description, how do you actually use them and are they helpful for someone who shots their AR every so often and not everyday?

A casual shooter should probably never need headspace guages.  They come in handy when building guns, however.  For instance, building FALs from old parts and a new receiver requires both timing of the barrel and adjusting the headspace.