Not sure if you're familiar with the "f*ck it, ship it" mantra, but it exists because any assumptions you're working off of and the ecosystem that you're building from is still considered a vacuum.
Think about flying an airplane. You're headed from Doha to Los Angeles and let's hypothetically say that your only form of instrumentation is ground control. Over 7,215 miles, you know the general direction of Los Angeles. At 560 miles per hour (execution, burn rate, build out, etc), if you don't check with ground control on how to adjust your bearing, you will veer off course at a rapid rate. Similarly, you think you know where the product needs to go. The only people who know better are your customers - aka ground control.
Startups require a great deal of clarity, then objectives, then execution. Constant feedback is a function of clarity. Without abundant clarity, the objectives will be off and you will be executing on assumptions.