The receiver gives you realtime data in the immediate vicinity. The exchange gives you slightly older data from everywhere.

Determining what benefit this gives the operator is left as an exercise for the reader.

You're right, but "slightly older" is on the order of seconds.

But only for those areas with receiver coverage that feeds them.

But your own receiver will always cover your area.

Initially I didn't realise historical data was available for free... I was also interested in learning more about the system itself, writing a diy decoder, etc... which is why I bought one. But yeah, kind of lost track of explaining that in the post :)