Why do many people have a word tab but I don't? Many people say that it is very practical so I would like to have one for myself.
The Words tab is an experimental feature being tested by Duolingo with A/B testing (one group has it and another doesn't); because of that one of your friends may have it and you may not.
But if you really want the Words tab, I suggest you make another account (you'll have to test out of everything though).
I think that words tab should be optional, so that people who want it can have it and people who don't benefit from having it can choose not to have it.