During the Waterloo blockade a few years back (and I expect it has happened since too, but I know for certain it happened then!), there were no trains serving the Chessington branch, and the replacement buses started at Worcester Park (whereas the trains branch off one stop up at Motspur Park).
Similarly, I have heard that RRBs serving the Worcester Park to Epsom stretch when that is closed, but the Chessington branch remains open, start at Tolworth. Geographically it makes perfect sense, but I've not personally witnessed it.
The reason the RRB don't go from Motspur Park is there is nowhere for the buses to layover and the road layout around Motspur Park is not that good, whereas Worcester Park is a lot better.
When the Surbiton resignalling was being done in the late 1990s, RRBs ran from Tolworth via Surbiton to Effingham Jct / Woking / Hampton Court with extra trains running between Chessington and Waterloo.
Also when there is work west of Feltham Junction the RRBs from Windsor / Weybridge / Woking / Ascot run to and from Hounslow, as there is plenty of space at Hounslow to park the buses, whereas at Whitton there is nowhere and the interchange is not very easy.
Additionally when the line through Twickenham is closed the RRBs run from Hounslow to Richmond or Barnes and not from Feltham, because the road layout between Feltham and Whitton is not a straight line and the distance from Hounslow to Whitton is a lot shorter.
I also remember when the River Crane bridge collapsed east of Feltham and there was a rugby match on at Twickenham that Saturday, trains were diverted to call at Kempton Park and the RRBs ran from there to Feltham. At other times the buses ran from Twickenham to Feltham.