Tellers, say Wikipedia, help their parties identify supporters who have not yet voted, so that they can be contacted and encouraged to vote, and offered assistance—such as transport to the polling station—if necessary. In as far as this increases turn-out, it can be said to be “good” for the democratic process, since a higher voter turnout is generally considered desirable.
In my time I have sat on wobbly plastic chairs / stood in soggy puddles / leaned on rough brick walls for hours-on-end in a multitude of polling stations across the land, at a variety of elections for all manner of Councils and Governments, as a teller, collecting voter numbers on behalf of a pick-n-mix bag of political parties.
And I can now confirm that the Friday 23 May UK election for the European Parliament was nothing like any other election I have been a part of. From the outset it was clear that voters considered this to be a second Brexit vote, not the General Election for the European Parliament, which it actually was.