I assumed that and I figured that it would be appropriate if you knew that the ball had to reach a specific, visible spot on the field (like the 12 yard line exactly) and your spot was beyond that.
Though not relevant to the play in question (which appeared to be a bad spot), one little referees' secret is that the spot is frequently marked exactly on a yard line in order to speed up the game for determining first downs.
You see it all the time, actually. It usually happens in the middle of the field after a first down, or after a turnover or punt, when the exact location of the spot is irrelevant. If the spot is close or very close to a yard line, the ball will be marked exactly at the yard line and the new series started exactly there, so that if any part of the ball crosses the yardage line exactly 10 yards away, they know it's a first down. Watch for this sometimes. Ask yourself why kickoff returns, interception returns, and some first downs between the 30s seem always to be marked exactly on the nose of a hashmark.
Here's the other dirty little secret about spots. The marking of where the drive or series is beginning is anything but an exact science. Did you ever think about this? When they measure for a first down, they are very fine about it -- bringing the chains out and measuring to see whether the ball is at the last link, etc. But how about when a first down is made and they set the chains? They basically have to eyeball it. The chains are 20 yards long. There is a marker that is on the chains exactly 10 yards from the marker stick. When a first down is made, the guys with the chains run forward, and the line judge tells them where the spot is, and that's where they put the 10 yard marker -- but this is never exact -- it can be as much as several inches off. It's the line judge's best guess after looking 25 yards across the field to where the umpire has spotted the ball. Once that's set, they throw down the orange marker, and they set the big pendant that's on the sticks so that it's exactly on the 5-yard line close to the first down, which is what they use to measure if there is a measurement required -- actually even that is not set exactly. Sometimes, they will even RESET the starting point. If it's a two minute drill, and they set the chains, and the first play is an incomplete pass or some other situation making it so that the line of scrimmage doesn't move (or only moves an exact amount, like a 5 yard penalty), the line judge has discretion to remark the initial spot and move the chains accordingly, if he thinks his original spot was off.
The irony of the exact measurement is that very few series, unless they start exactly on a yardline (e.g., a touchback) are exactly 10 yards.
