Each individual army, so long as it keeps the same units in it, will gain a passive buff over time, eventually adding up to...i dunno...+5% att and spd. If units are switched out, the buff for the new units is removed, the units moved out of the army is removed, and the units that stayed buff is reduced by one level (and it begins to slowly rebuild).
Why? Two reasons:
1. Passive buffs are awesome and make everyone feel warm and fuzzy inside.
2. As an army in real life works together and familiarizes itself with each other, the army as a whole tends to become more efficient and effective in their tasks. Apply that here and it adds a bit of realism and a lovely buff!