Question
Why do we need to weed a soil?

Answer
I'm not sure if this is a serious question -- it is short enough -- but on the off chance you really want to pose this, I'll go ahead and answer.

Weeds obviously grow places we don't want them.  But there's more not to like than that.  According to recent research, most Weeds form chemicals of varying toxicity and inject them into the Soil around them to get rid of the competition.  We've only recently learned about this and many of these chemicals have been identified; their effects are still being studied, but we know they aren't toxic to ALL competition, just some of it.  The chemicals can be made in the leaves, and distributed in the Soil when the leaves die and decompose.  Or they can be secreted by roots and poured into the Soil around the plant.  It's an area of growing interest, and little is understood.  We'll know more in 5 years.

Hard to believe, but a Weed in your Lawn or Garden can be producing chemicals that make your more favored uncomfortable.  On the other hand, Weeds aren't the only plants involved in chemical warfare; perennials and trees do a lot of that, too.

THE LONG ISLAND GARDENER