It's not magic, it's simply that you can pull off more tricky shots with the lighter stick.
If I have time to set up and I'm not tired, I can hit a much cleaner heavier ball with a 13oz 370 swingweight racquet. Check back in the 3rd set and see how I'm doing. And obviously, heavy racquets are much better at handling big serves and heavy spin and pace, but most of us don't face enough of that consistently to warrant using a 13oz+ frame.
When it comes to scambling defensive shots, little slice drop dinks, lobs, etc...everything is easier as you go lighter.
My racquets (NXG OS) actually weigh close to 13 oz's out of the box strung with an overgrip and dampener, but they are so head light that they actually feel and play lighter than an even balanced racquet that is an ounce lighter.
Exactly the same for me. Although my heavier racket is a 12oz and like you, with time to set the shot I too hit a cleaner, more penetrating shot, but not every point is like that and sometimes my 11oz racket can be much more effective at defensive shots like a stretching lob or reaction volley.
Its one of those catch 22's whereby Im probably about 75% effective with both, but there are always certain shots that I wish I had the other racket in my hand sometimes.
If I'm honest to myself, I probably need to improve my technique with the lighter racket so I hit the penetrating shots with that as well as I can with the 12oz racket. Whilst I love the 12oz racket, I'm not always convinced I have the technique to use it effectively.
This forum clearly has a lot of very good players and its easy to perhaps get a bit unitentionally intimidated into thinking that heavy is the only way, so I do like to hear about good players using lighter sticks. Particularly with talk of tennis elbow etc, but surely that is down to poor technique as much as anything.
I do like reading the positive posts from users of lighter rackets, to me lighter is a strung weight of 290 to 310g, and I always like to read and learn how they play and make their rackets very effective. I like to play an all court game and volley a lot, having grown up watching the golden era of the likes of McEnroe, Edberg, Cash and then Rafter.