Its because the bulbs used are only supposed to run 1 element at a time, when you hold the lever halfway you get both filaments on simultaneously, ie. 120w bulb. The big bonus is that main beam points into the distance, and your dipped beam points low, so you get good all round visibility.
If you have a quad grill, then when you flick main beam on, the spots point low too (same effect as above).
Its possible to re-wire the lights pretty easily so when main beam comes it, it keeps dipped on too, just need to use two extra wires and a diode and jobs a goodun. My mate runs this on his Corsa, he drives from Oban > Glasgow a lot on tiny unlit roads, so has wired up his lights to give him the best visibility - obv. at the risk of melting the headlamp, fire, blinding someone blah blah.
I've got 100w bulbs and spots, plenty bright for me, but when I put the 2 lap grille back on (quad only goes on for winter), I shall get a cheap HID set (yes I know its illegal, blah blah, uprated loom blah).