First not all altcoins are useless as there are good altcoins there in the market. Secondly, yes we need altcoins as each (that is, only those with good use case) have something to offer, and these are the ones we need in addition to Bitcoin to disrupt this current era.
Of course altcoins on the market too much, but many of them in recent years have grown into a noteworthy project, and some will soon be fighting for the top lines, the development does not stand still and such a struggle is useful