When should you select text?

Before making any action on text in Microsoft Word, whether formatting it, cutting or copying it or deleting it, you must tell the computer exactly what is the specific piece of text that should be affected by the action.
Whether you want to format or delete a single word, or a certain line, a paragraph, or even the whole document, you must tell the computer: "this is the piece of text that I want to imply my action on".

For example: you want the words read this to be bold, therefore you must select these two words prior to applying them a bold format.

Another example: if you want the spacing between the lines of the whole document to be "single spaced", you should select the whole document, prior to applying the new line spacing.