Like mikesmith said its just easier and allows you to keep your cadence up. Shifting is faster no pause while you change the front mech as well up or down so you can get back on the power without having to make a second shift. I also found I had more useful gear range closer together. One less thing to manage on the trail has to be more efficient and make the trail more enjoyable IMHO.
Granny gears are more of a prop than useful and how often did I use them or my top gear in 3 x 9? Not very by the time I had changed to outer then shifted up I would be looking at down shifting for next climb etc Fire roads or tarmac es but then thats not what I want to ride well or at all if I can avoid it ha ha
Why not book a demo or mates bike with 1 x 11 and try it out? That has to be the best way to decide without a massive outlay?