These guys are part of a personal security detail (PSD), and their job is to protect VIPs from embarrassment, harassment, and harm.
While you probably don't work as a personal security agent as your day job, if you're a man with a family, you've still got some VIPs that you're responsible for keeping safe: your wife and your kiddos.
The world is an unpredictable place. While you and your family are unlikely to find yourselves in the middle of a dangerous emergency, crimes and accidents happen, and random, civilian-targeted terrorist attacks are statistically on the rise.
A man's most ancient role is to act as protector for those he loves. The skillset needed to fulfill this calling has changed from time period to time period, but the charge has remained consistent. It's a job that need not involve paranoia, becoming overly cautious, or loading up on tons of "tactical gear." Rather, it involves adopting a calm, but vigilant mindset — a state of relaxed alert — and carrying a few tools that are better to have on hand and not need, than need and not have.