A dedicated golf GPS watch is very accurate. Depending on how much you spend you can get various features but all will give you the yardages to the front/middle/back of the green. Obviously they aren’t going to tell you to the flag as that moves daily. I regularly play with a bloke who is a slave to the scope and my watch tells me pretty much the same as his scope.
I used to only use Mk1 eyeball and I am pretty good at estimating yardages but the GPS watch is that much more accurate and allows more precise club selection. You can assess the yardage by eye and select,say, a 7 iron but when you use your GPS you change your mind and select an 8 iron. This club lands on the green...the 7 iron would’ve been off the back. It can make a difference although,In the end,it comes down to the individual golfer having a reasonable idea about how far they hit each club and then executing their swing. Easy then!
Another advantage of GPS watches are if you are playing a new course. You can eyeball the yardage (as discussed) but when you are playing your home course you get used to yardages and associated landmarks. On a new course (or one you only sometimes play) you don’t have those reference points. Admittedly there are usually 100 yard markers or similar but that restricts you to a fairly narrow band. Yes,you can buy the course yardage guides,but they aren’t cheap and they soon add up!