Is there a reason to use 她是一个老师 instead of just 她是老师?

What is even the meaning of 一个

There are so many sentences where there's no need to add the
一个 in them . Ealier there have been translations like , 她 是 老师 Then why the need to add that here. When Singular subject means its one entity ..

But what if she isnt?

It was not explained in tips!

Why not * 她是一老师 * ? Literal meaning, She is a teacher

Adding 一个 is superfluous and not often used by native speakers.