Simply, m6ths are dominant 7ths (actually 9ths) with the dominant's 5th in the bass. It's called voice leading. An indirect form of back-cycling. 4th or 5th bass movement rules. It's the essence of jazz, rock, country, pop or American. Just throw out the m6, it's a publishing ruse to make music more complicated. A very common useage is "Stairway to Heaven." It's simply a writing preference.

Ma6 is a fill. It's a I variation or a I vi, with I (or i) in the bass. I just don't look at keys anymore. It's a means of notating. Which honestly, is how most music is written, nowadays. You have to know your stuff, before you can be hired, especially in lucrative studio session playing.

One can look at this in many ways. Be open. I steal from very innovative players. My knowledge didn't come without very, very hard work. You may see something new in just, simplicity.