I like chronies for two reasons. If you don't know how fast your bullet is leaving your rifle, you're guessing. Since I like to hit stuff farther out, I want to know where I can expect my bullet to be when it gets there. If you change the actual velocity in your ballistics calcs to 200 fps faster or slower, you will be surprised at the difference out at 400 yards. I don't want to guess at yardage either, so I use a rangefinder to tell me exactly how far away the game is standing. The less guessing the better.
The other thing the chrony does is let you know how consistent your load is in velocity. Deviation of only 10 or 15 fps for 5 shots is common for loads I settle on. It is a comfort knowing the cartridge you chamber isn't way slower or faster than you think it is.