Not a lot in terms of direct spend, but they pay APD and taxes (depending on route and stopover times) and as many of them fly on British carriers (not that there are too many of those left now) there is an indirect benefit.
Think APD is limited as its a transit
Also, the friendly and efficient welcome they get at the major airhubs might encourage them to come back for a holiday or business trip ..... or maybe not!
Dunno as few airports I have transited through have encouraged me to go and visit the city.
Yup there are some I will or have visited but transiting through ORD has made no opinion on me wanting to visit Chicago etc.