I can't say specifically why she values using Word as her text editor of choice, due to not being in her head. I know she uses some features such as adding notes to pieces of text (which I know she uses to track what she needs to work on/revisit later, etc.), but I don't know how many of those features she uses or how many advanced features she touches.
I suspect a large part of the reason she sticks with it though is threefold: familiarity, volume of content in the Word format, and working with agents/publishers. (like another commenter mentioned) She's already familiar and comfortable with using Word, and so she can be productive and just get things done by sticking to what she knows. (she has expressed displeasure with the idea of upgrading due to possible UI changes) She has a /HUGE/ amount of stuff already written using Word, and moving that to anything else would be a herculean undertaking which is, honestly, not worth her time. And using an automated tool to attempt to do so could lose information, which she is already extremely protective of. Lastly, because she needs to work with other people, and might submit documents to those other people in a digital format, it basically needs to be in Word, otherwise that person is unlikely to ever even touch her stuff. Having to deal with agents as a writer is harsh, as they won't even tell you if you did anything wrong. They'll just ignore you. Submitting an unknown, uncomfortable format to those people will just cause them to ignore her anytime she tries to submit anything.
While you might consider plain text to be the ultimate format which is compatible and usable everywhere, people who are not tech people (my wife is not very "techy") just want to get things done with tools that are highly supported, comfortable for them, and widely recognized and used by the people they need to interact with. While you might consider VIM/Emacs/Sublime Text/<insert plain text editor here> to be the ultimate editing environment, anybody in the world my wife deals with will just give you a blank stare the moment you try to deal with those people using those tools.
I suspect a large part of the reason she sticks with it though is threefold: familiarity, volume of content in the Word format, and working with agents/publishers. (like another commenter mentioned) She's already familiar and comfortable with using Word, and so she can be productive and just get things done by sticking to what she knows. (she has expressed displeasure with the idea of upgrading due to possible UI changes) She has a /HUGE/ amount of stuff already written using Word, and moving that to anything else would be a herculean undertaking which is, honestly, not worth her time. And using an automated tool to attempt to do so could lose information, which she is already extremely protective of. Lastly, because she needs to work with other people, and might submit documents to those other people in a digital format, it basically needs to be in Word, otherwise that person is unlikely to ever even touch her stuff. Having to deal with agents as a writer is harsh, as they won't even tell you if you did anything wrong. They'll just ignore you. Submitting an unknown, uncomfortable format to those people will just cause them to ignore her anytime she tries to submit anything.
While you might consider plain text to be the ultimate format which is compatible and usable everywhere, people who are not tech people (my wife is not very "techy") just want to get things done with tools that are highly supported, comfortable for them, and widely recognized and used by the people they need to interact with. While you might consider VIM/Emacs/Sublime Text/<insert plain text editor here> to be the ultimate editing environment, anybody in the world my wife deals with will just give you a blank stare the moment you try to deal with those people using those tools.