Think of someone helping Grandpa with using his email. On the phone, most of the time is spent in back-and-forth, explaining a concept multiple ways until he understands the request. The information he gives back may be vague and incomplete, so you can discuss it with him to get instant further clarification and context. You can then walk him through the solution in a bi-directional conversation, whereby you say a step and then he asks for clarification about certain aspects, before you proceed to the next step.
Now consider email/text/etc. Focus even on the solution stage. You send 5 steps to perform to complete the task. He gets stuck on step 2, and so responds. He doesn't know if he'll get stuck on steps 3, 4 and 5 either because he can't get past step 2. That's potentially 8 emails back and forth to get the problem resolved.
By not having instantaneous bi-directional communication, it is a hell of a lot harder to debug and walk someone through the solution particularly if they're not technically adept. For us on HN it may be fine - we can stumble our way through blanks relatively easily - but for Grandpa, he has no hope of doing that and so phone support is by far the most time-efficient method.