F2P are not profit by themselves. But they might eventually go for a subscription later. Most of nowadays new subscribers would not have started playing if it was a subscription from start (or even worse pay 40€ for first month like it was in the beginning).
F2P also make other new F2P or subbed players stay because the would is more alive and has more low-level players to play with. WG is not Blizzard that can ask people who have never played their game to spend any money just to try it (actually D3 has a free, capped and area limited trial too... but it's over after an hour per toon).
F2P also make other new F2P or subbed players stay because the would is more alive and has more low-level players to play with. WG is not Blizzard that can ask people who have never played their game to spend any money just to try it (actually D3 has a free, capped and area limited trial too... but it's over after an hour per toon).