From a theoretical standpoint (music theory), most chord embellishments (like 7ths, add9s etc) are there for color and beauty. The basic Maj-Min-7, and dim are the building blocks for the melodies that float through the (chord) changes, but adding the "color tones" is what separates the simple from the sublime.
Example:
A simple I - vi - lV - V7 progression will give you a suintable backing for thousands of songs. Much of the 50s and 60s music in the US is loaded with this very chord progression. If you add a 6th to your l chord - it's almost the same as your straight vi chord. In C, the notes of a C maj are C-E-G-C, and a C6 is C-E-G-A. Notice that these notes are teh same as an Amin7: A-C-E-G ... this is where your bass notes become paramount to the tonality you're seeking.
I won't go into a big theory lesson here, but the watered down version is this:

1) To any Major, or Minor chord, you can add a 2, or a 6 without changing the fundamental integrity of the chord. The added notes serve to sweeten the harmonic structure, and add some live to an otherwise basic chord.

I'll address your questions in detail if you have any, but this is the simplest way to add love "sizzle" to your basic chording.