I know as someone new to business it can sound counter intuitive, why would you want to sell to less people? But this simple answer is:
Yes, no matter what business you're in, you need a market segment.
Defining your market helps you focus your business, and tells you who you need to be as a company. If you're trying to market to everyone, your company ends up looking like it has multiple personality disorders, and for both investors, and customers, this is a red flag.
Having a well defined market segment helps you be efficient, consistent, and will help save you a lot of headaches in the future, the worse thing you can do is set out in your business with too broad a market, and have to start from scratch again later on.