What is important about it? Dorking has 3tph to London, stations between Effingham Junction and London Road have 3tph to London. Bookham is a relatively small village on the outskirts of Leatherhead.
You do realise the service doesn't exist just to serve Bookham, but even so, Bookham is not a "small village" as you claim anyway, its just very a spread out area, and with the incredibly poor service, many who live in the area choose to be dropped off at Leatherhead or just drive all the way to their destination instead. Ironically, Effingham, Horsley and Clandon would all perfectly fit the description of "small villages," yet they all have a much better service than Bookham due to the railway geography in the area, and even more ironically, Effingham village is actually closer to Bookham station than it is Effingham Junction. Going further into the realms of speculation, I think there would be a lot of merit to opening an additional station on the line in Fetcham (roughly where the railway goes over Cobham Road), which would bring much more of the area between Bookham and Leatherhead into walking distance of a station. In addition the purpose of the service via Epsom is not to link Effingham Junction, Horsley, Clandon or London Road to London, as this is the purpose of the faster service via Cobham.
What you have completely ignored is the fact not everything is about getting into Central London, the main purpose of the service is to link Guildford with Leatherhead, Ashtead and Epsom, as well as for onwards connections to Sutton and Croydon amongst other places. There is not an insignificant level of inward demand to Guildford, with it being a large, important town within Surrey, with many employers, lots of leisure activities available and a large university (bear in mind many commute to university, they don't all live on campus!), as well as the station providing lots of onward connections towards Portsmouth and Woking amongst other places. There is also an awful lot of inward demand to Croydon as well as smaller amounts to Sutton, which this service provides connections to, and additionally lots of school journeys to areas around Sutton due to the number of "selective" schools in the area. To this end, I'd be in favour of extending the London Bridge to Epsom services to Guildford (not stopping at Horsley and Clandon though), and sending all 4tph SWR services to Dorking (bringing me on to my next paragraph). Journeys between Bookham and Waterloo (as well as the small number of journeys between Guildford and stations between Ewell West and Raynes Park) would be possible with a cross platform change at Epsom, which is not an issue when the services are running at half decent frequencies. A 2tph service to Bookham with a cross platform change onto a 4tph service onwards would be much better than now.
In regards to the frequency, the SWR service is by far the most used service south of Epsom, as it is the fastest service to Waterloo and Clapham Junction, as well as also serving Wimbledon, a place with lots of inward demand as well as connections to the District Line, and having only 1tph to Dorking is wholly inadequate, and even 2tph to Leatherhead and Ashtead is inadequate I would say. Yes 4tph to Dorking isn't completely necessary as I suggested in the previous paragraph, but it is necessary as far as Leatherhead, and Dorking is a convenient place to turn around. The even bigger issue being missed is that anything less than 4tph is completely and utterly inadequate between Epsom and Waterloo at any time of the day. These services are very overcrowded as it is to the point where they lose time and leave people behind, and this is totally ignoring the swathes of people that have given up on the railway and now use alternative routes or modes of transport for their journeys. Before you say "the number of commuters has reduced" bear in mind that there was 6tph between Epsom to Waterloo at peak times pre-Covid, and nobody has suggested that those additional peak extras return, as the slightly reduced number of commuters now means that 4tph is (just about) sufficient.
This ridiculous, poor attitude on display is exactly why the railway is failing passengers and why car usage is not going to decrease. We should be trying to induce more journeys and decrease car usage, not cutting services to the bone.