It really depends on the system. If we're talking AD&D, things like THAC0 charts, weapon speed modifiers, saving throws are useful.
If we're talking D&D 5e or PF2, things like condition descriptions, DC charts, descriptions of skills. If there are special properties for weapons/armor, that is useful. Range bands for ranged weapons.
What's never useful to me are things like random NPC charts, rolling for random events. That's stuff I do before the game, and if I need that during the game, the DM Screen won't have enough information to do it correctly anyway.