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)?
Now, there are two ways to niche: you can niche vertically by industry or horizontally by skillset. And if you do one, you can do the other when you're ready.
For example, if you start with a vertical: if you've worked with several different industries, you know which you enjoyed most and which you enjoyed least. Whatever you enjoyed most makes up your niche. Now if these verticals are polar opposites (restaurants and government), but you don't want to choose among them, go horizontal by picking one set of services you enjoy and creating a profitable package suited to each vertical. Now you've created your own unique niche with a single specialty service for a handful of verticals you want to focus on exclusively.