Actually decoys are very useful in Ukraine Russian war. It is usually decoys of air defense or long range precision fires like Himars and target is to waste resources of opponents long range fires which are limited and/or expensive.

Further more you can also reveal position of the attacker and counterfire.

If you have 500 tanks and 500 cardboard tanks, someone with only as many real tanks as you have may not bother attacking. Thus, having the cardboard tanks saved you a battle.

If someone with 1000 tanks attacks, it's a battle you would not have won anyway.