For quarterbacks, for example, they offer the following:
- QB Rating: an adjusted NFL QB rating which accounts for dropped passes, spikes, balls thrown away, and yards through the air with some other adjustments.
- Accuracy %: easily calculated elsewhere, but the formula for this is ((Completions + Drops) / (Attempts - Throw Aways - Spikes - Batted Passes - Hit As Thrown)) to give an idea of how accurate a quarterback actually is without outside forces.
- Deep Passing: provides stats specific to passes 20+ yards in the air, including TDs, INTs, Acc. %, drops, and attempt %.
- Quarterback Passing Under Pressure: shows a number of stats when the quarterback throws under pressure including a breakdown of the type of pressure, pressures as a percentage of dropbacks, sack %, completion percentage, and acc. %.
- Play Action: provides passing stats for play action plays side-by-side with non-play action plays including the difference in completion percentage and YPA.
- Time in Pocket: provides average time until sack/attempt/scramble as well as the difference in passing statistics when the time to throw (average of time to attempt/sack/scramble) is above and below 2.5 seconds.
Hell, if you examine individual player profiles, they'll even give you breakdowns on passing to different parts of the field.
That's just for quarterbacks. They have other useful statistics such as elusive ratings and breakaway percentages for running backs; deep passing, slot performance, and yards per route run for wide receivers; pass blocking efficiency for offensive lineman; run stop % and pass rushing productivity for defensive linemen; and many others.
There's a wealth of information beyond their initial grading that really makes the extra money worth it. The grades themselves are a very small part (but the most easily marketable) of the information that they keep.