In my opinion??
I think the greatest thing about Warhammer and Warhammer 40k (as their appeal is fundamentally the same for me) the fact that you get to place your persona/will/attitude into a table top force and battle against others who are doing the same. You clash against opposing will's, crush the attitudes of your lessers, and get destroyed by the persona's of your betters (just to give it a bit of dramatic flair)
For example, I play Imperial Guard in 40k. I enjoy the "everyman" aspect of the army and how its power is in its massive, unending numbers and firepower. To grab a quote from the 5th edition Imperial Guard book:
"I have at my command an entire battle group of the Imperial Guard. Fifty regiments, including specialized drop troops, stealthers, mechanized formations, armored companies, combat engineers and mobile artillery. Over half a million fighting men and thirty thousand tanks and artillery pieces are mine to command. Emperor show mercy to the fool that stands against me, for I shall not.
? at the outset of the Salonika Crusade, 733.M38"
THAT is just badass, and (from a gaming perspective) when you get together with like minded people who can get into the spirit of their particular faction/army/race (whichever you prefer to call it) it really brings a new level of enjoyment out of the game.
I explain it to my non-table top friends like this and the fact that, sometimes the lore/rping value doesn't matter to some people, and that is totally okay.
Hope that helps.. and remember
"Victory is achieved through mettle.
Glory is achieved through metal."