Before you launch, yes (ish) - it can give you an idea of where you'd like to start charging and why. Models come into their own post launch. They provide a clear structure on how to optimise the economics of your business. E.g. If you are currently selling at $20 and losing $3/sale due to support costs and returns, it provides a great structure to focus on a) increasing the price, b) reducing support costs / order and c) reducing returns.

A dream for any VC is a startup with fantastic economics at day 0, but this is rare. The majority of high quality companies have negative unit economics during their infancy. We liked investing in companies with negative unit economics with founders that understood the drivers of their economics deeply and were optimising them aggressively on a weekly/monthly basis (and doing all this inside of a H-U-G-E market). (The best ones did it on a weekly basis.)

An interesting example is Just Eat, one of Europe's best performing startups (IPOed at £1.5bn). This company had negative economics for ~3 years, but the market was very big and investors could see a pathway to positive unit economics through optimisation. This allowed them to fund the company through the negative UE period.