Digital's absolutely right about knowing the range. Check a trajectory chart for your favorite cartridge. You'll find that, if the range is 150 yards, and you think it's 100 yards, doesn't make a lot of difference.
If you think it's 450 yards, and it's 400 yards, or 500 yards, it makes a huge difference!
At the long ranges, where a 50 yard range estimate error is easy to make, being off by 50 yards means you miss, or worse, break an animal's leg.
Rangefinder's the only way to fly.