First of all, I think that the title "product manager" is not a very good one, and is actively harmful. While it is technically correct, the word "manager" in most people evokes the idea of managing people, and there is a very short step from there to seeing PMs as ones who "manage" everyone involved with the product development. "Product owner" is not much better either.
But we don't have a much better term, so PMs it is.
For all practical purposes, the amount of features a software product can have implemented is virtually unlimited. In reality, features can only be added over time, and the role of product managers is to guide the prioritization by making product decisions. Long time ago, in a talk at a startup event, I compared PMs with the people being quartered in medieval times [0]: the product development process is pulled in different directions by various "horses" - groups more or less directly involved in the development and use of that product, and the PM's role is to balance those horses and prevent any one of them of dragging the whole thing in its direction. (I have identified those horses as, respectively: sales, marketing, end users and engineering. This is a common arrangement, but others are possible in some environments.)
Ultimately, the PM should have enough understanding of particular interests of each of those groups to be able to make informed decisions. Sometimes, the priority will be to make the product easier to sell (e.g. add this feature this potential customer is asking for); at other times, the priority will be to resolve some technical debt to make deployment faster; and so on. Obviously, that understanding does not come from vacuum; it requires talking to the representatives of the all of the "horses" and gathering all the necessary information.
In practice, it is common - as some other comments have mentioned - for the PMs to succumb to the pressure of the "business" side of the equation (sales and marketing). As a technical lead, I have always made a point of working together with the PMs and help them keep that balance.
[0] https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0...