If the world doesn't trust you, you won't sell a thing. PR is a tool for you to create and improve your business' credibility. When people hear about you from third party sources they trust, they'll trust you more.
PR is not just about traffic. Improving trust and credibility multiplies the core metrics of your entire business. Media placements (articles, blog posts, TV, radio, or podcast interviews) send bursts of targeted, qualified traffic to your website. Linking to your news coverage on your website improves your conversion rate (on both your website and in your sales pipeline). And more credibility means your customers are more likely to buy more expensive services more often.
An investment in PR is not just about improving your sales and marketing funnels. It impacts the rest of your business as well.
You need credibility to raise investment rounds and get financing from banks. The more credibility you have, the more investors you'll be able to attract, the more you'll be able to raise, and the higher the valuation you can justify.
Credibility also makes it easier to build teams. The best talent wants to work for the best companies. More credibility attracts more high quality talent, and decreases your employee turnover rate.