Pareto Principle, commonly known as 80/20 priciple, goes like:

80% of outcomes come from 20% of causes

I have experienced the impact of this rule many times in my life. After gathering the requirements, if I venture into building 100% of the requirements, it usually takes forever (in startup timeline). Where as if I prioritize, I am usually able to deliver 80% of the product with 20% of the effort. This is a huge win. In a startup, usually we are fighting with limited amount of resources to deliver something in the shortest amount of time. If we deliver 80% of the requirements in a short time, customers can play with the product and provide early feedback. Sometimes they may not even need the remaining 20% of the requirements. 80% may be good enough.

Striving for perfection or even 100% is pointless. Good enough is good enough in 80% of the cases.