My go-to example is Lemlist (and their "Lempire" of related products). The founders built their own cold email agency, grew that to millions just through sheer labor, then built software to automate that process and now they're at a 200 million dollar valuation from more than 10 to 15 million dollars in annual recurring revenue. They have lots of marketing channels: organic SEO via content marketing, Facebook group with frequent events like interviews with other agency owners and salespeople, YouTube videos that are then recut and repurposed to TikTok/Instagram/Facebook videos, partnering with influencers, paid advertising, and of course, cold messaging other salespeople and agencies via LinkedIn and email. Most technical founders do not know or have the patience to do any of this. If they do, though, they could similarly grow to such an ARR in a few short years, as Lemlist started only in 2018. Similar examples include GoHighLevel and ClickFunnels which are almost at or over a 1 billion dollar valuation each, both bootstrapped. The ClickFunnels founder for example sold his software via live demos (both webinars and in person events) where you got X% off but only if you booked right at the end of the demo, which apparently drove quite a high percentage of their initial sales.
I'd highly advise founders to start a marketing agency if only to understand marketing in a real capacity and how to deliver results to paying clients. I have done so via a cold email and paid ads agency for various niches before and I believe it's helped me immensely with future products. Best of all, you learn to be truly aligned with your customers and providing them direct value because they are paying you every month. If you make a mistake, you must fix it, you have no backup fund of venture capital previously raised in order to skate BY, especially now that ZIRP (zero-interest-rate phenomenon) is over.