Simply put:
BEFORE departure, despatch landing performance (planning performance) is to be taken into account, to ensure the flight can be completed to its destination, OR if unable two alternates (eg a new destination plus diversion to an alternate) must be valid before dispatch with associated fuel requirements.
AFTER departure, enroute landing performance (inflight actual performance data) is taken into account, to ensure the flight can be landed at intended destination.
Intended destination in the latter paragraph may well be the original destination if actual conditions are better than forecast conditions which prohibited its planning use to be suitable.
For instance:
Due WIP in GDN a reduced runway length exists and forecast weather conditions are SN with runway state reported at planning stage to be braking action medium.
The above reduces you planned maximum landing weight to conditions which prevail and the safety margins built in to them, thus well reducing your payload capability.
Operational experience has shown the runway will most likely be treated by the time you arrive, but no guarantees, thus you cannot assume this and dispatch performance is thus defining a payload reduction.
Your flight will now need to be using a second alternate with no restrictions for planning performance calculations, so all payload booked can be taken.
Once departed and approaching destination, the crew can access actual conditions in GDN and establish if Inflight Performance is suitable to make an approach at the scheduled destination, win for all parties involved, passengers at destination, aircraft in place for return trip, no diversion requirement.