A local website has this table for calculating the size of downpipe
needed for various roof sizes. The roof size is stated to be the "plan
area" . Why should the roof pitch be relevant, assuming that the rain
falls roughly vertically?

Maximum plan area of roof served by downpipe (m2)

degrees Roof pitch
0 - 25 26 - 35 36 - 45 46 - 55
65mm Round 60 50 40 35
80mm Round 100 80 70 60
100mm Round 155 130 110 90