When to use fan?

Apologies if this is a really basic question - I don't play Genji at all, full disclosure - but I have been watching a lot of Genji players lately and I have been really curious about the use of his secondary fire: I have noticed a lot of the time when engaging from mid-to-long-range, Genji players will mix in secondary fire shots with primary fire, but ostensibly this doesn't seem like it would do anything but hurt their DPS output; I understand the basic principle of fan being better for finishing off enemies at close-range due to its ability to fire all three shuriken at once, but from checking the Overwatch Wiki, it doesn't appear that secondary fire travels faster nor does more damage per-shuriken (in fact, it appears that it actually lowers one's rate of fire, and thus lowers DPS), so it seems strange to even think about using it from a range where only one of the shuriken is going to hit at a maximum.

Can somebody please help elaborate why players do this? It has really been puzzling me!