Perhaps, but it doesn't HAVE TO BE intrinsic. A unit may want to attack a specific unit, in which case he selects to move on the unit. Or he may want to move to the place the other unit is, in which case he clicks on the space. But what if he clicks on a space and then another unit moves there? Would he want to attack or just move there? Or would he want to run away? Sometimes he doesn't even see that other unit before the order is issued, other times he doesn't know the unit is going to move there, or just hopes it won't.
This situation comes up A LOT. By setting an "attitude", we can resolve these questions very easily most of the time. With an aggressive attitude, an attack will take place (assuming they're at war). With a passive attitude, an attack will not take place. With a normal attitude, an attack will take place only if your unit is stronger; if he's not, he runs.
Defense too could benefit from attitudes. WHen an enemy unit tries to occupy your unit's space, an aggressive defender will stay to the last man, but hopefully inflict extra damage. A passive defender will run away first chance they get, hopefully taking no damage. And a normal defender will stay until say half its strength is gone, at which point it will try to run.
Because we don't want to bog down combat by stopping it and asking for input (and indeed, there are plenty of times in Civ where I would've LOVED to give some input into a combat to save an important unit), we have to give them orders to carry out, and hope the situation doesn't overwhelm those orders.
Because OMFG, chasing down units sucks! There's this little game called Power you can find at the Underdogs that uses simultaneous movement, and you have to chase guys down with much larger forces because if you don't, they capture yours. The only thing that saves you is the thing that annoys you the most - you can only make 5 moves per turn.