Public Relations is a tool in your marketing toolbox to use for creating awareness, attempting to control the public narrative, and solving certain problems. There are all kinds of ways to use PR but you should start with asking yourself what kind of problems you need to solve and then figuring out if PR is the best way to do that, as opposed to assuming you need PR and just waiting for the "right time" to start. It is very hard for a young company to use PR for growth marketing unless there is something truly newsworthy or interesting about the product, company, founders, clients, etc.
Some examples of different ways to apply PR:
You can use press releases to create awareness for the company's launch, a new product or feature announcement, new investment, a new strategic partnership, or anything else you want to announce. Press releases can help you get content out on the web that talks about your features and has backlinks that can drive SEO value. Most press releases will not automatically secure coverage in any good publications unless it involves a large newsworthy company. However, a recent official press release may add a feeling of credibility to your company when potential customers search for you.
Securing press in actual publications is much harder and very rarely has to do with issuing press releases. It requires having or establishing a relationship with journalists or bloggers and adding value to their lives by making it advantageous to them to write about you. That could be as simple as you being very relevant to their audience, giving them a discount for their readers, or building a relationship long before you ever ask for anything by simply giving them inside data or scoops about the industry you're both in. If you can help them write a great article with tons of data from a paid industry survey you conducted, they may return the favor when you have news to announce.
I have used PR about my funding round to attract other investors. We offered a tech publication the news of our funding as an exclusive story. Once they published it, which they would not have if we had used a press release, I used the article to send to other investors.
I have released a press release about a partnership and then emailed the release to clients asking them if they saw our partnership with XYZ company. The brand name of your partner will often increase your credibility, as will the press release, even though you paid to publish it, and it gives you a timely reason to reach out and re-spark a conversation with people in your pipeline.
I have very rarely had a good experience with a PR specialist. There are way better ways to get press if you decide to focus on them, but they involve grassroots outreach usually from the founder.
Feel free to touch base to chat about PR for startups.