A founder posted a question in a startup chat group recently:
My cofounders and I are all intimately familiar with our industry. Do we need to do market research?
I firmly believe you should. In particular, the best research to do is direct research. Since customers are going to buy your product or service, don't you want to talk to them before building anything?
Wouldn't you rather spend a few weeks asking questions of potential customers before you devote five years of your life to a new business?
I certainly would.
This customer development time also helps with fundraising. Investor types see that you have some hustle. They also realize that this investment is a bit more de-risked because you have done your homework.
There's a caveat: Don't fall into the trap of pulling out some Gartner or similar market sizing stuff. If the investors don't understand your market is going to be huge, they're likely not investors you want, anyway.