Have a look at the two pictures below. The top one shows an inward opening rear door with a drip moulding fitted. The drip moulding is fixed to the bottom of the opening part of the door (the door sash) and is there to deflect rainwater over the thresh. Without a drip moulding, water can run straight onto the rubber seal (gasket) and find it's way to the inside of the door frame. During heavy rain the rainwater can flood the thresh of the frame and leak into the house causing damage to flooring.
In fact all doors should have a drip moulding fitted, but it's an easy thing to miss off during manufacture to save money, so if you look at upvc doors as you walk down a street, half of them probably won't have one.
The bottom picture shows the same door in Ashington after Lockex fitted a new drip moulding to the door. This customer was complaining of water ingress making the carpets wet at both the front and rear doors.