Competent home inspectors will always attempt whenever possible to get up on the roof. Often it is not the roof or more accurately the shingles that are of interest, but various other areas that require scrutiny.
For example masonry chimneys especially need a thorough looking over inside and out. Any penetrations through the roof and associated flashings need to be examined. Seams where the roof and home join are always of importance. And don't forget those gutters.
Arguably some of these "hot spots" can be examined from the ground through a pair of binoculars or the edge of the roof. But not all of the areas of interest can be seen well enough with these methods to make a good determination on their condition. Unfortunately some roofs are not accessible due to height, slope or weather so the roof edge or the binoculars may be the only means of inspection in those instances.
I was on a roof recently that looked to go on almost forever. Walking around up there was quite a work out!
My advice is whenever looking to work with or hire a home inspector ask if they walk the roof. If they answer no you may want ask what other parts of the home are not examined.