Every golfer needs to know the yardage on most shots they play. Some golfers get that data from a caddie. Some use laser-measured readings on sprinkler heads. Most, however, use a GPS or a laser rangefinder to get those yardages.
However, most golfers would be shocked to know that, technically, under the Rules of Golf, they're not allowed to use either a GPS or a laser rangefinder to play.
That's because the USGA and R&A consider GPS units, GPS apps on your phone and laser rangefinders as distance-measuring devices (DMDs), which provide information above and beyond what a fellow competitor could discern using what's available on the course to help them figure out yardages. For example, every golfer can use the numbers on the scorecard or step off yardages from plaques or do some computation with a pin sheet. However, since every golfer doesn't have a GPS or laser rangefinder (though they should), golfers who have them technically have an advantage.
This means that if you're playing with a legal eagle for fun (that doesn't sound like much fun to me), they might try to get you on a penalty if you whip out and use your GPS or laser rangefinder. And if you use one in a regular USGA competition, then you would be breaking the rules, too. Specifically, you would be breaking Rule 14-3. The penalty for breaking that rule in a stroke-play competition is 2 strokes for the first violation, then disqualification for subsequent violations. In match play, it's loss of hole for the first offense, then disqualification for continued violations.
However, in 2016, the USGA amended Rule 14-3. They now allow golfers to use distance-measuring devices in competition if a course or organizing body creates what's called a local rule -- basically, a special rule for that course or competition -- allowing golfers to use their DMDs. The caveat is that the DMDs can't also measure things like slope, wind direction, a player's physical state (like heart rate). They also can't offer tips, advice or recommendations, like a caddie might.
Most golfers don't compete in these kinds of events or tournaments, so they don't have to worry about local rules. So, go ahead and use your phone app, GPS or laser rangefinder. Even if it has slope or wind functions. If it helps you play faster and enjoy the game more, go for it.