Guild Wars 2

Because you can still apply quickness directly to other players without using SoI. Tides of Time will grant quickness to allies that it hits, Well of Action will grant 4 sec of quickness on the third pulse, and if you use it, TW will grant quickness each pulse. Boon duration affects all of these sources of quickness for allies

OriOri.8724:

Because you can still apply quickness directly to other players without using SoI. Tides of Time will grant quickness to allies that it hits, Well of Action will grant 4 sec of quickness on the third pulse, and if you use it, TW will grant quickness each pulse. Boon duration affects all of these sources of quickness for allies

This.

Also SoI only copys boons active on you. Boon duration helps with this. Besides that though, as OriOro said, to increase boon duration provided from non SoI sources.

Not sure what you guys are on about, but the change to SoI a few months back explicitly made boon duration affect the boons it provides.

Prior to the change, SoI was only a copy: it copied the durations without regard to your stats. Now, SoI functions as a boon generation skill with a conditional of what boons it provides. SoI scales off of boon duration just like anything else.