Your cognitive capacity is reduced significantly if you are in the presence of your smartphone, even if it's turned off, new research has shown.
... it didn't make a difference whether their phone was on or off, or whether it was face up or face down on the desk. The mere presence of a phone, in sight and in reach, reduced participants' ability to focus and perform tasks because their brains are constantly working in an active manner not to pick up the phone. [emphasis mine]
So that's why I hate that thing so much! I just have one so a few important people can reach me, but I always manage to leave it somewhere as far away as possible so it can't bother me.
I have pockets. The phone only comes out if I need it in order to look something up for the work I'm doing.