In the NFL, 3 and 7 are the biggest key numbers (~15% and ~9% of all game margins). Moving a spread from -3 to -2.5 is much more valuable than moving from -5 to -4.5. Sharp bettors pay attention to half-point moves near key numbers, especially when shopping lines across books.
Advanced concepts
Key numbers
The most common margins of victory in a sport. Landing on or through a key number shifts EV significantly.
Example
NFL: 3, 7, 10, 6, 14 — in that rough order. NBA key numbers are much less pronounced (margins spread smoothly).