What are we “future proofing” with plain text?

One of the things I always read when reading about people using plain text files and something like Markdown (or other similar markup languages) is that they’re “future proofing” against some time when nothing can be read but ASCII files.

Today I ran across Beorg, a plain text-based time management app that uses “org mode” as it’s chosen plain text format. One of the benefits listed is:

Future proof. View and edit beorg files in any text editor.

Future proof what? What exactly am I gaining by using plain text in a task management application? Are there really people out there sitting around worrying about whether old or completed todo items can be read decades from now?