When it comes to a website, you need to focus on a single niche and not be all over the place. You will need to really buckle down and not stray from your niche because it could cause confusion with your dedicated viewers. If they come to your website or blog because they love all the content about online marketing, don't post about politics. You don't want to be seen as an "All over the place" website or blog because no one will know they should come to you for specific information.
If you don't stick with a single niche you'll likely be seen as one of those fake news websites. Being viewed as a fake news website will make it so you're seen like a fake, obviously, and not an authority. You may be able to convince Google and the other search engines that you're an authority through your link building, but the people will know you're all over the place.
Why should you stick to just one niche?
Besides what I covered above, you will want to stick with one niche per blog or website because you will want to target all the traffic that it can bring in. If your random traffic comes in through one article and can't find anything else about it on your site, you're likely just pushing them away. By keeping your content niche specific you will increase the chances of the user clicking to a different article on your page and this will increase the chances of them landing on a sales page or clicking through an ad you have placed on your pages.
What if I'm a news website?
If you're a news site you can usually target different niches, but you can't get too technical when it comes to certain things. Lets say your post is covering something about SEO in the world today. You won't get into details on how to build a backlink with html hyperlinks because it just doesn't make sense for your readers. They want to know what you're talking about but they don't want to know every single little detail as to what exactly is going on. Give them bullet points along with some content but don't get so into it that you push them away. Not everyone wants to get an in depth review of how something works exactly, and you'll need to remember that.
Can I target slightly different niches?
Yes and No. Let's say you're in the SEO niche and want to target web design, you can do this because they're basically related. If you're posting for a pluming niche you won't talk about the type of car or van you're using because that will pretty much be worthless. No one wants to know what you're driving as long as you're doing a good job for them. Post about plumbing services and what you can do for your clients for a specific price and everything will be good
Sticking to a specific niche will increase everything across the board. You will be seen as an authority in your niche, you will increase your engagement and you will also increase your conversions. If you're all over the place when it comes to your website, your visitors won't know why they're there other than they article that brought them in. Stick with a single niche and you'll be happier later on down the road. I've seen blogs post about anyting that is in their head and it's never a good blog or website lol. They're all over the place and it's super confusing, so stick to your niche to not confuse people when they're tying to figure out what you're about.