even to maintain a single dev of a bootstrapped app one needs to recoup its costs, and for that you need users, so there's that. more users is always better especially if users themselves drive your growth since server and development both cost so you start up pretty well in debt.

you have also to consider that solution don't grow in a vacuum. the moment you build something, you can count of having ten competitors out there doing the same or similar thing, akin to convergent evolution someone else is experiencing the same need you're solving right now and among those someone is building a solution. if you want to monetize your solution, unless it's a physical thing, you need to be the first out there and the faster growing.

additionally, if the strategy involves financing at some point, a big cache of user waiting/registered is good leverage and something you can build without having a product if you have the resources to do both - more financing, more money to grow and outgrow competitors.

people sometime demonize growth but unless your startup is something that inherently doesn't scale growth and lock-in are practically the first filter to weed the competition.

It’s not wrong, but if you have the opportunity, why not generate interest and get customers as early as possible? We’ve had success convincing institutional customers to agree to purchase even before we've built the product. You get validation and revenue, and it's a great story for investors and partners.

You may simply build a product that nobody wants.

As opposed to building a product that lots of people want, but can't use because it didn't scale? (Scaling costs money)