There are several reasons why the additional platforms at Queen St work, particularly if there are similarly 2 platforms available at Central in each direction.
Firstly, it enables a second train to enter whilst the first is still in the station or just leaving. This in itself is a big improvement, particularly given dwell times at Queen St will not be short in the rush hour. It also enables a second to enter if the first is unable to depart north because of a junction conflict.
Secondly, it reduces knock on delays at the junction. As an example, if a train from Cathays is unable to cross the junction because of a northbound Heath train, but by the time it is able to, a southbound train from Heath is in arriving, the delay will be reduced to solely the time taken for one to clear the crossing and enter the station.
Thirdly, it distributes the passenger flow better, and reduces the crush on the platforms, which in itself can have implications for dwell time.
Fourthly, if a service has a problem whilst in the platform, the whole service doesn't get backed up.
The last piece of the jigsaw in improving capacity is reducing dwell times by having adequate length suitable stock so that boarding and alighting is fast. Sadly, the Welsh Government has yet to play its part in the improvement of services in that regard.