Somehow I must have joined it but why? Is it legitimate or some scam? If legitimate what does it offer?

Poserworld is legitimate, but I don't know how current its content is.

Hmmm?🤔 

It is a site that sells legitimate Poser content. It is all older content, though.

Hmm. So they may have stuff that poser has stopped selling?

You will need to own all those older models to take advantage of the site.  Trying to convert clothing is not an option unless you have the skill to perform it comfortably.

The latest figures they support are Dawn and Dusk. They aren't selling stuff Poser stopped selling. By Poser, I assume you mean Renderosity or Poser Software. They make and sell their own content. Much of it is very old. Their props might be of more use currently than their clothing until you get comfortable enough with Poser to use the fitting room.

I buy some things at Poserworld occasionally. As previously said, it's mostly older stuff but I can use CrossDresser to convert V3/M3 clothes to newer figures.