My second game with Ogres (or was it my third?). Centerpiece of the army is a unit of 7 Bulls, Tyrant and Skullmantle/Siegebreaker Butcher (deployed 3x3, total about 800 points).
Vs. Dwarfs, before their new book. Dwarf player has about...3-4 war machines and the rest is the usual Clansmen/Stubborn Hammerers/Crossbows/Rangers mix.
My big unit runs forward like a herd of...well, a herd. Turn 2 his shooting knocks enough models off to force a Panic check.
I run backwards 12".
I rally on Turn 3, run back up on Turn 4. Turn 5 he causes another Panic check.
I run backwards 12".
800 points of troops never make it into the opposing half of the battlefield. [Actually more like 1100, since whilst running backwards the Tyrant's unit caused some Panic checks on other Ogre units around it.]
Another example - Game 1 of some RTT way, way back. My Orc army is centered on 24 Black Orcs and a BO Warlord - figure one unit is worth 500-550 points. Playing Lizards. Turn 1, Chameleons march block me and I'm still only halfway out of my deployment zone. Turn 2, his Stegadon marches up to within 6" of my Black Orcs. I take Terror test. I fail. I run 12" off the board. [And the annoyance factor spurred me to wipe the Lizards out in the next 4 turns.]
A final example - not mine. RTT, long ago. Nurgle Minotaur army facing TKs. Minos are led by 7 Minotaurs + Doombull (600+ points). TKs double-fire Skulls of the Foe at the unit on Turn 1. Panic test failed. About 30% of the Mino army (and the only unit with a standard, plus the only damage-dealing character...) runs off before Turn 1.
These are many other examples, but basically a big unit of expensive models a) hits hard, and b) hits its _owner_ hard when it's lost due to a tactical error, freak chance, etc. The epitomy of this is a Dragon or a Greater Daemon army. If the Dragon/Greater Daemon dies (say, to a lucky cannon shot or three), you've just lost a third of your points and possibly much of your hitting power. On the other hand, if you can keep the thing alive, it will typically do major damage.