If you move a unit with low remaining movement points on top of an army/unit with an empty slot, they will form a single army.  All units in the army will have their movement points reduced to the lowest common denominator.  

Examples:

Lets say you have a retreating unit who stops its turn on another army with an open slot, your other army just lost its ability to move/act this turn.

Lets say you have a summoner who makes a 2 movement point elemental in an 4 to 5 movement point army, you move the elemental out to dispatch a pioneer or something, but your high movement point army now has only 2/5 movement points even though none of the units in it have moved this turn.

Another example.  You start your turn, and see a target 3 moves away.  Your army has a mix of 3 move and 2 move units.  You decide to just attack with your 3 move units.  You won't be able to because your 3 move units will start the turn with 2/3 movement points.