When you need to make something seem very important, bold seems to be the right choice.

Italics can be used for foreign words like the Greek týpos — „reflection, form” and gráphō — „I am writing” which form typography.

It is rather rare to use underlined text, as this stands out very much and should be used with extreme caution.

There are also situations when we need to remove something, for example during collaborative editing. The strikeout is a right choice then.

If you are dealing with software development, having the option to mark the inline code like printf("hello, world\n"); is also very useful.

These are also subscript and superscript types that you may utilize in chemistry or in math related texts where you have things like H20 or x2.