Many of my goal based investments got derailed because I’d sell impulsively whenever a stock hit multiple upper circuits or some news popped up. Looking back, secondary would’ve been a game-changer. 
Quoting Nithin’s tweet:
When I was trading actively (before Zerodha), I had an offline demat account where I held all my investments and an online account for all my trades. This was a way to avoid the temptation of “trading” my investments
If I had to sell any of my investments, there was effort involved. I had to physically fill out the delivery instruction slips and send them to the broker to move holdings to my primary demat and then sell them.
The other advantage of having a secondary demat was, of course, taxation. If you have both short-term and long-term holdings in one demat, First In First Out (FIFO) taxation applies. Check the link in the comments to know more.
We wanted to offer a secondary demat for a long time, but there were some challenges, and we finally figured out how to do it. You can now open a new demat and use it to segregate your long-term and short-term holdings and smartly manage the tax impact on your holdings. This is a good behavioural hack to deal with impulse selling.
Unsurprisingly, the best returns were on stocks that I held the longest in my secondary demat.