Why do people buy things? Why do users use a new app? Is it because it solves a need they have? Does said app have to solve a specific need?
If the answer is yes, then it makes sense to be really sure that need exists before committing to building something. But what if the answer is no?
What if users use a new app only because it's become true that other people like them are using it?
Instagram never really filled a need, rather it becomes cultural. It became a thing that your friends use. A way of connecting with the people that matter, but it's only useful because they also use it.
How would you possibly test, before you build Instagram if Instagram is going to be the thing people use?
This isn't just limited to big apps with millions of users. How about expensive headphones. We had headphones for a long time. They were good, you could hear the music without any problem. Then having really good headphones became a thing people cared about. Before people cared about it, there wasn't the demand for the volume (pun intended) there is today. Now many people care about good quality headphones to listen to their music, but why do they care more now than they did in the past?
Is it because that they see other people, people they think are similar to them caring?
When you build an app, or create anything, what makes it successful? is it filling a need, or is it potential users realising that using you're app is now to sort of thing people like them use?