A single tech stack can only teach me a limited amount. When I build something around an unknown-to-me language/framework/tool over a weekend, I usually walk away with new insights.
For example, playing with Meteor (js) was the first time I was able to understand the concept of pub/sub.
That knowledge can sometimes be brought back to other tools I use, sometimes not. When it cannot easily transfer, the new tool gains brownie points. Playing with the "flavor of the month" taught me something useful. If I stayed with the same established workflow I always use, then I wouldn't have had a reason to encounter pub/sub.
For example, playing with Meteor (js) was the first time I was able to understand the concept of pub/sub.
That knowledge can sometimes be brought back to other tools I use, sometimes not. When it cannot easily transfer, the new tool gains brownie points. Playing with the "flavor of the month" taught me something useful. If I stayed with the same established workflow I always use, then I wouldn't have had a reason to encounter pub/sub.