¿¿ what is <meta charset="UTF-8"> and do we need to use it ??

Answer 1

That tag is to set the file to be able to recognize special chars or accents in words like "cómo" without needing to use the ASCII code for an accent for example. You don't need to use it but it's easier for the programmer as well as someone trying to read you file instead of having many ASCII nomenclature.

It's a standard format so almost everybody uses it.

UTF = Unicode Transformation Format

Answer 2

It gives instruction to your web browser to encode the file with UTF-8