Branding and image. The GTI logo is iconic and VW were making a statement when they launched the mk5 GTI (first decent FWD Golf for 14 years) and using the globally known image to create niche sales using the GTI logo on things other than the car (bit like football teams do, they don't just sell tickets and replica shirts), watches, clothing etc etc.
VW also do this with the Polo GTI
SEAT do a similar thing with the Cupra models
Audi ditto with S models
And yes, they do it on the mk6 GTI too
When the GTI image goes off the boil again they'll probably revert back to having Golf GTI on the back of the GTIs using the Golf's iconic image to prop up niche GTI sales and so the wheel will continually revolve until we're all driving electric powered plastic boxes
I'm sure other marques do similar things too only I wear VAG blinkers so don't care.