The most important part of your business is actually doing the revenue well. That's not up for debate. But all the good work in the world means nothing if you can't get anyone to pay attention to you. Whether you're selling things or providing services, you need to get in front of people, and one key to that is search engine optimization.
When people are looking for something specific, they go to Google to try and find it. You want to go where your customers are, so that's the perfect place to meet them paid search ads are one important part of this, but SEO can be just as valuable for getting customers.
Users don't click on many ADs
Google is very specific about separating ads from the rest of its content. You probably do it yourself, skipping past the ads to look at the "real" search results. While paid ads will get you at or near the top right away, a lot of users will never consciously notice them, and will click on organic search results, so being at or near the top of those results is far more valuable than paid search advertising.
Organic traffic comes with purpose
Like mentioned before, prospects coming to Google have a purpose in mind. They're trying to find or get something, and you want to be in front of them when that happens This is different from social media or other advertising, where you're forcing your way in front of the customer. Both are valuable to have, but a potential customer with a purpose is much further down the funnel to being an actual customer.
Own your sites
You already do a lot of marketing on social media, so why do you need a website? That's a job in itself, so what's the point of putting in all the extra effort to make a website and keep it at the cutting edge?
You don't own Facebook, Instagram, Twitter, or any of those other platforms. You don't even really on your page or profile with them. You do own your website, so if there are any large changes to one of those platforms, your website will be unaffected.