In marketing it's important to know who you're targeting so you can find them and create your brand messaging around them. In some cases the niche is in front of your eyes - in others not so much. I'm specifically thinking of B2B or freelance work - imagine a designer, or a marketing expert - how do they decide who to market to? Every single industry/company size or pretty much every business could use their services.. but how do they pick a niche to focus their efforts on, if they've been basically been working with a really wide audience and filtering on the basis of budget (like most in these industries)?
Choose a niche where:
1. You enjoy the work.
2. You already have some satisfied clients.
3. There is an ecosystem of related professions (ex: realtors, bankers, title companies, lawyers, home inspectors).
Number three is important because solid businesses are built on a pipeline of referrals, and clients won't give referrals to competitors.
So, for example, if you are a designer focused on real estate agents, don't ask for referrals to other agents. Ask for referrals to real estate lawyers, home inspectors, bankers. The agent would gladly give them to you. Then, build relationships with them, saying that you design websites for real estate agents. They refer you to agents and you can refer them to agents. Then, when you get new real estate agent clients, ask them for lawyers, inspectors, etc. So it becomes a virtuous cycle.
Also, if you hesitate to pick a niche, remember that a niche is for outgoing marketing only. If people from other niches come to you, then feel free to work with them. So you really aren't limited to the niche, its just that you market that ay.