Most computer users use Microsoft Word. That is unfortunate for them, since Word is proprietary software, denying its users the freedom to study, change, copy, and redistribute it. And because Microsoft changes the Word file format with each release, its users are locked into a system that compels them to buy each upgrade whether they want a change or not. They may even find, several years from now, that the Word documents they are writing this year can no longer be read with the version of Word they use then.
But it hurts us, too, when they assume we use Word and send us (or demand that we send them) documents in Word format. Some people publish or post documents in Word format. Some organizations will only accept files in Word format: someone I know was unable to apply for a job because resumes had to be Word files. Even governments sometimes impose Word format on the public, which is truly outrageous.