We have untagged tag with 7 questions tagged.
Is it necessary to have this tag?
if yes, why?
The untagged tag is automatically generated; there's a cleanup process that runs which deletes any old tags of a certain age (I believe six months) which have only been used once. In the rare case where that low-quality tag was the only tag on a question, it will be replaced with "untagged" (since the system doesn't let a question exist with no tags at all).
A question should never be created with the untagged tag; if a proper tag doesn't actually apply to the question itself chances are it's a poor fit for the site anyway. Untagged questions will crop up from time to time though just through automatic processes, the proper procedure when one comes across them is just to figure out which tags should apply and retag it.