I started out playing 10-finger and did so for many years. But I had injured my left pinkie finger one weekend doing something and the next round out it hurt like hell gripping the club. It hurt so bad during my warmup session that I was left with two choices - either go get a refund/raincheck and go home or try a different grip.
And it couldn't have happened at a worse time... I had a tournament coming up in two weeks, and I desperately wanted to play. So out of mere desperation that morning, I switched to an interlocking grip, and that seemed to do the trick as far as the pain was concerned. Except, of course, every other shot went opposite the direction I was used to experiencing. Oh well... I guess I'll just have to deal with it.
Although I struggled that round and the next few rounds after, I started feeling more comfortable and got used to it. Not only that, I noticed that I was hitting the ball a little higher, and I also noticed that the occasional hook that would seem to occur when I really went after the ball on certain shots (like when I was between yardages and took the shorter club and tried to get a little extra out of it) - the hook wasn't happening. In fact, for the first time in years I was seeing a nice high fade with my mid-irons that landed softer and closer to the flagstick instead of coming in lower and rolling out further away from the hole.
So that alone convinced me to continue using the interlocking grip. I'd somehow stumbled across the cure for that unexpected hook, albeit because of that pinkie injury, that had my hands working in unison on my takeaway. It helped me to get a little more width and verticality in my swing, and prevented my left hand (I'm a lefty) from overpowering my right hand on the downswing and impacting the ball with a closed face.
That subtle change completely changed my game, and although I struggled in that tournament two weeks later - I finished that season hitting 3 more greens on average per round. It was huge for me.