6max is great for so many reasons.
1) More hands per hour
This instantly improves your $/hour, even if your BB/100 stays constant, which is always nice.
2) Get to isolate weak players more often
In FR games, it's more difficult to get to get to heads up flops just due to the law of large numbers. There's a greater chance another player has a hand to take a flop with you, or just decides that they want to.
3) Get to own the button
In FR, you have to have 6 people fold consecutively to get able to really steal the crap out of the button. Again, by playing with a smaller field, now only 3 people are ahead of me on my button, so once they fold I get to open with ~55% of my hands, more if I've table selected properly and have passive blind defenders on my left.
Overall, you just get into profitable situations more often, and get to exploit those situations more as well. Combine this with getting dealt 70+ hands per hour instead of 50+ at FR, and it's tasty
C9 has a solid point about hands per hour. If you get to the point where your decisions are automatic, esp at tiny micros like $5 or $10nl, then you can get a lot of hands per hour at FR. If I'm playing FR at $10nl, I'm 16 tabling, while at 6max, 6 is the most I'm comfortable with. Any more than that and I'm not making solid reads, etc. and I'm more focused on improving my game and stepping up the levels.
16 tables was getting me around 830 hands per hour, and 6 6max tables gets me 403