This is one of my favorite topics, and I write more extensively about product-led marketing versus sales-led marketing on the Product-Led Growth Collective site: https://www.productled.org/blog/marketers-prepare-product-led-growth
The tl;dr, though: In many product-led growth (PLG) companies non-salespeople (e.g., product managers, designers, engineers, founders, etc.) helping to create the actual product have the initial greatest influence on what product marketing does. In many PLG companies, product marketers find themselves in particular helping out product managers with research, positioning/messaging, launches and adoption marketing.
In sales-led companies -- or traditional software companies -- the sales leaders have a much greater influence -- than in pure e-commerce PLG companies -- because they’re bringing in the revenue. Not some PLG e-commerce engine. In sales-led companies, the product marketer is doing everything they do in PLG -- research, positioning/messaging, launches, and adoption marketing -- and they help out the sales team with sales content, new-product training, and more.
Today's most difficult job for a product marketer, I believe, is working in a PLG company that’s going "up market" because you have three masters: product leadership (who ran the show when they only way to buy was with a credit card), sales leadership (who is trying to influence company direction to close bigger sales deals), and of course whomever your boss is. :)