Q:

Why are there so many tags which are not used?

Why are there so many tags which are not used?
Are they not helpful? In such a case why do they exist?

A:

Some of those tags are tag synonyms. When those tags are used on questions, Stack Exchange automatically rewrites to the standard tag.
In other cases, 1 question has that tag. When only one question has a tag, it is not displayed with a count.
In some other cases, no questions have that tag and it is not a tag synonym. In that case, the tag will automatically be deleted at 0 UTC each day.

