Whilst a monocoque may not feature a separate chassis, you do still require items such as chassis rails, floor pans, centre tunnels and sills (rockers) which are all welded together to form the monocoque.
If you look specifically at Volkswagen, take for example the Mk3 Golf (as I know these cars fairly intimately) the engine and gearbox sit on a couple of bolt up cross members which form a kind of sub-frame.
This sub-frame is virtually identical to the one found in the previous Mk2 Golf and Corrado, it is identical to the unit found in the MK3 Jetta / Vento and is also found in the Seat Ibiza along with several other cars. Indeed the whole substructure of the front end of the Mk3 Golf is identical to that found in the Mk3 Jetta / Vento with only cosmetic panels and trim items such as bumpers, grill and headlights being changed to give the appearance of the two cars being different.
The same is true of components such as brakes, suspension, wishbones, CV joints, etc, etc, etc... The wining looms are also based on a common design.
If you look at more recent cars such as the Mk4 Golf, this again shares a majority of underpinning parts with the Mk4 Jetta / Bora and the Audi A3.
Move up to the Mk5 Golf and you find that the Mk6 Golf is again using the same substructural design, as is the Jetta again. The Passat uses the same sub-structure up until the middle of the car where some panels are elongated to give the car a longer wheelbase, alternative wishbones are used to give the car a wider track. You can then look at cars like the EOS and see something which is based on a Mk5 / Mk6 Golf substructure (i.e. floor pans, sills, wheelbase, etc) but uses Passat wishbones to give it a wider track and improved handling. The latest Scirocco also uses the same Mk5 Golf "platform" but with restyled bodypanels and a lower roof line to give the car a more sporty appearance.
The latest Mk7 Golf uses something called the MBQ platform which is the latest cross-car platform and incorporates spaces for things like batteries for electric version, etc...
Platform sharing reduces development time as designers can concentrate on the styling of a new model without having to worry about how it will handle or how it will perform in crash testing. Also, dealerships can train their mechanics on a single platform and they are then able to service all cars that use that family or generation of platform.