Using styles you will not need to change each tag every time when you want to change the text font. If you are making a single page site
then you can define style within same file within head tag. If your website has more than one page and you want to use styles on different pages, you need to define external styles. These styles can be used in each documents. In future you just need to change the main file and the relevant texts would be changed with this one time change.
Using Classes to Define Styles