1st possibility:
I flew F100, 421 Ktas & 60 min TaT and now I fly E-Jets, 455 Ktas & 50 min TaT. So I can't just transfer them through the scheduling page, would lose and awful lot of time with planes sitting idle and/or land at awkward times.
So consider the manual option of editing the route. I change the plane, but the TaT doesn't adapt, so I also have to change it. One more (extra-useless) click on hundreds of pages.
2nd possibility:
I open a bunch of tabs for new routes for the 737. Then I'm out of planes and decide to put the thinner shorter ones on E-Jets. Once again, the TaT doesn't adapt, so I also have to change it.
Dani used the Q400 and 767 only to underline heavily the difference, could be 2 different planes that would work well on a same sector.