The thing is you wouldn't know if such thing is useless until such time you buy and already use it. Most of us are easily tempted to buy things that are best sellers because we see people are using it but later we just realize that it is not good for us.
Also people are easily attracted to those products sales for up to 80% especially in the malls or branded products.
They didn't know that it's only a strategy for any warehouse to make it sold out their product batch so they can proceed on selling the other set of batch.