Details and data can be conveyed more easily textually than verbally.
I can prioritize responding to emails, but phone calls demand urgency even if it’s unwarranted.
It gives me time to organize my thoughts.
It saves you from a) having to take notes while we talk, or b) not taking notes and then asking me the same questions again two days from now.
I don’t have to talk to you … I mean, it’s nothing personal – usually, but I’m an introvert so if I get to choose between talking on the phone and not doing that, 9 times out of 10 I’ll pick the latter.