Hi Shazam
it's playing the game better than leaving the army in province, and losing both the troops + ressources
at least, you limit your losses and force the attacker to lose time + ressources (army maintenance costs) going on a wild goose chase
the next phase would be to not only leave him nothing to plunder, but also forcing him/her to stop before he damages your fort (and you have to spend stones to repair it)
the 3rd phase will be how to ambush the attackers and score a defensive win, thus not only damaging his army, but also earning ressources
and finally, the last phase would be destroying his army + going to the counter-offensive and plundering his kingdom where he has now become your own private bank
Have fun,