Why should I cover my vents?
You can see in the photo the evidence of water coming in through the vent bringing mud and debris along with it.
More evidence
The marks on the wall are caused by water seeping through the porous concrete
Water damaged posts
You can see where water was at on the wooden post. Even though the water is gone that wood absorbs the moisture, causing it to rot and weaken.
Looks gross
The residue left behind by ground water seepage looks gross, I wouldn't want anyone in my crawlspace looking like that!