One role of the PM is to close gaps between departments and make people work together toward the same goal. It is very hard for a manager to do the same thing. For a PM it is easier because he is not the boss of anyone and don't try to gain power over other's stuff. On the other side if a manager is trying to do the same thing, you will have other managers try to protect their department.
As for Apple, yeah they are one of the most successful of the planet, but the feedback I have, is that developers would love to have PM to work with.
The issue is not to have PM or not. Anyone company in the world understand the benefit of having a good PM. But a lot of PMs are shit and just make things worse.
It's a little more nuanced. Apple doesn't just throw away the PMs and tell eng leaders "You just have to do that work". Their culture has baked in the whole "DRI" concept where a single individual is directly responsible for all aspects of the product, and has corresponding decision-making responsibility and accountability for its success or failure. In typical companies, when eng and PM disagree, the decision is often escalated up the management chain, to someone even less involved and clued in. With the "DRI" model, any conflict between eng and product needs is purely in the mind of one person, and gets resolved right there.
The tough part about doing this is when you hire engineers, you need to also interview for good product sense and have more emphasis on soft skills. Not easy to find.
As for Apple, yeah they are one of the most successful of the planet, but the feedback I have, is that developers would love to have PM to work with.
The issue is not to have PM or not. Anyone company in the world understand the benefit of having a good PM. But a lot of PMs are shit and just make things worse.