It is entirely possible to have a long, successful guitar career without much musical theory knowledge. But, theory is great to know for a variety of guitar-related tasks and activities, such as songwriting, teaching, and accompanimental playing.
When first learning piano, or any instrument for that matter, you want to learn the names of the notes on the staff. Knowing the note names will allow you to further your skills with music theory, sight reading, analysis, and so on.
The most common and most useful of these chords is the seventh, which you can use pretty much anywhere just to add a slightly different color to a chord, or in some types of progression to give the sequence of chords some extra thrust, because the added seventh is a mild dissonance, or clash, that makes the chord sound like it wants to move somewhere.
Chromatic alterations, such as sharps and flats, can be used to help create a more interesting chord progression in a piece of music. Ed Bell explains how to use these alterations effectively.
Musical modes constitute a form of scale complete with their own distinctive melodic traits. The seven modes have their roots in some of western music’s oldest forms.