I remember when I wrote up my diploma thesis using word. I was working on the document both in university and at home. For some reason when opening the document at home, word decided to reverse all opening brackets < into closing brackets > and vice versa in my formula. That's when I started using latex.
Using latex means that you are basically "programming" your document. This also means that it is quite easy to change the style of the document, e.g. when you want to resubmit some paper in another journal.
Typing of text, special characters, formulas etc. is extremely quick as you are coding it using basic ascii characters and you can use all the nice help tools for working with code like e.g. regular expressions. It is true, word has this in rudimentary form, too, but it doesn't work well.
Finally with latex you learn a lot about good typesetting, e.g. to use a typographically correct "minus" in front of numbers. To do that in word you have to somehow insert each time the unicode character 2212, if you don't like to use the equation editor for each number.
I usually use word for short, simple or informal texts, while I would never use it for a scientific paper or even thesis.