He's relevant to 40-50 year old women who pine after their heyday following Take That and go to all his concerts to throw their underwear on the stage for "our Wobbie!!".
Slightly hyperbolic assessment I admit but it seems mostly true.
Frankly I find the man immensely irritating and never ever got the appeal even when I think back to my youth in the 90s. He can't sing live, he talk sings most of the songs or indeed just gets the audience to fill in most of the lines. Now some have then said "oh but that makes him such a good performer"...ahem, Freddie Mercury was a star live performer, he let the audience sing parts of the song, but man could he actually sing.
Secondly the guy dances, as he even has freely admitted himself, like a drunk dad at a wedding.
Finally, and probably not entirely his fault, the media (mostly ITV to be fair) then makes the assumption the whole world lives "Our CHEEKY CHAPPY ROBBIE". No, we don't.