That extract from RTT - WTT posted by JonathanH is fascinating.
Before February this year there were only 3 SE passenger services into Blackfriars and all are shown on the extract. These were the 0707 and 0807 arrivals from Ashford International via Maidstone East and the 1739 departure to Ashford International via Maidstone East. A little further back there were two evening services IIRC - one at 17XX and one at 18XX.
These services are effectively ancient relics of the time (before 1900) when the London Chatham and Dover Railway had Holborn Viaduct as its City Terminus and Victoria as its West End one, whereas the South Eastern Railway had Cannon Street and Charing Cross. The creation of the Chislehurst curves after the LCDR and SER merger created the option to vary between all 4 terminals, as JonathanH points out, but the main reason these 3 services still don't use the Chislehurst curves and go into London Bridge and Cannon Street is, I believe, capacity constraints.
Under the original Thameslink proposal there were plans to run two Thameslink trains an hour from Maidstone East, via Chiselhurst, London Bridge, through Blackfriars, Farringdon and on to Cambridge. If this had been implemented as an all day option, it would probably have killed these 3 services, but it failed (again on capacity grounds) to materialise.
The cancellation of the Maidstone to Cambridge service has caused a lot of consternation in Kent, and as a possible partial sop (a very cynical one IMHO) to these feelings, SE has (from February this year) started running off peak Maidstone-Blackfriars trains. Basically they turn the 0807 arrival around and send it back to Maidstone at 0832. It then works back to Blackfriars and returns to Maidstone at 1140. I think it gets replaced by another unit in Maidstone which comes up from Ashford (e.c.s.) and then returns to Blackfriars to work the 1440 to Maidstone. This finally returns again to London in time to work the 1739 back to Ashford International.
The time-honoured morning and evening trains are well patronised as they are the only direct fast services to the City for passengers on the (ex-LCDR) Swanley-Maidstone East-Ashford line. Before COVID the two up trains could be packed leaving Swanley. However, from casual personal observation the new off-peak ones seem to be running virtually empty.
What is fascinating about the RTT extract is all the e.c.s workings - surely the ones from Sevenoaks are TL not SE. Could it be an error ? And if you look at the occupancy of Platform 4 between the arrival of the 1335 from Maidstone and its subsequent departure to Maidstone at 1440, there is an e.c.s. that also uses Platform 4 between 1402 and 1409. That doesn't make much sense either. Any ideas ?