Just like a lot of things we use in this industry it was originally used for what Gilly said. Measuring paint thickness on cars. They can be had for really cheap or over $1,000. I've tested mine up against a $1000 unit and it was very accurate. Ours is a different model than Pierre's but I bet they both work equally well. If you use the glisten method and have proper speed and pressure with the scoop coater you'll be in the right range for EOM but we still use our meter all the time. I test our coated screens so I can see if my guy is doing them right without having to sit there and watch him. I find screens all the time where he forgot what coat he was on and either over coated or under coated a screen and I let him know about it. I can also tell when he's coating too fast. The EOM meter won't necessarily make your prints better but it will help dial in a lot of variables that you may or may not be doing right. But with it, at least you'll know where you are and what you need to do to get things to their optimum level.
The way I look at some of these tools is whatever gets us great quality prints with the least amount of work possible is what I'll use. Now my squeegee durometer tester doesn't do much for the overall operation but it didn't cost much so it is just a cool toy to have. The loupe, tension meter and thickness gauge do get used quite often around here.
