Law Enforcement: A holstered handgun on an officer's hip provides an immediately accessible tool to protect both him or herself and the public. Revolvers were dominant in years past, but semiautos in 9mm, .40 S&W, .357 SIG, with a few .45 ACP and .45 GAP, predominate today.
Military: Many military personnel have duties that preclude them from carrying a long gun. This includes pilots, medics and corpsmen, transport drivers, crewmen in tanks, and others. They are often issued a handgun for personal protection. Some Special Ops soldiers carry one as a backup and as a primary gun for some close-quarters combat missions.
Personal Protection: Handguns dominate the civilian personal-protection market. As the saying goes, "I carry a concealed handgun because I can't carry a cop, and a rifle is too heavy and attracts attention." This market is dominated by compact revolvers and semiautos in .380 ACP, .38 Special, .357 Magnum, 9mm, .40 S&W, and .45 ACP.
Hunting: A handgun has less range than a rifle, but more than a bow. It's not easy to take game with a handgun, and many relish that challenge. There are revolvers and single-shot pistols that are capable of harvesting any game animal in North America and many African species as well. Some are equipped with scope sights and are capable of cleanly harvesting big game at more than 150 yards.
Survival: A compact .22 LR revolver and a 50-round box of cartridges take up almost no room in a pack or kit bag. And if you're traveling a long way from nowhere, and things don't go quite as planned, that little gun may just supply enough small game to keep you fed until help arrives.
Woods: If you're wandering around woods that are home to bears, mountain lions, wolves, or wild boar, a big-bore handgun on the hip can be quite comforting, and comfortable to carry. You may never need it, but if you run into a big critter that decides you're lunch, it's nice to have a .44 Magnum, .45 Long Colt, .454 Casull, .460 S&W Magnum, or .500 S&W Magnum handy.
Competitive Shooting: Competition is hard-wired into human genes, and handguns are an excellent choice for those who love to compete. The numerous games involving handguns are accessible to nearly everyone. They have led to some useful innovations in handgun design that benefit other users.
Recreation: "Plinking" is the common term used to describe shooting a handgun for the sheer enjoyment of bouncing a tin can or pine cone across the ground, or seeing how close you can come to the bull's-eye. There's no signing up for a match, no personal protection, no hunting—just make the can dance or hit the target center. It's therapeutic and fun.