It’s true, there are plenty of engineers who are capable of dealing with customers. PMs, however, did not come from an idea that engineers are antisocial, you’re making assumptions. PMs have long existed in other industries where the very same is true as software; the people building the product are often capable of talking to customers, but don’t.

The reason PMs exist is because it’s a full time job, just like most roles. Being an engineer and/or tech lead is also a full time job. In larger companies, we tend to split out full time jobs. Often in small startups, people have multiple roles. So in a small company you might see someone talking to customers and engineering. In a large company, it’s far more likely that the engineer and the PM are two different people.

The work involved in keeping up with a roadmap, convincing execs that the roadmap is worth investing in, communicating it with the marketing teams, content teams and support teams, tying it back with customer demands and needs, and all that really ends up taking most of your time as a tech lead, and you no longer have time to work on tech design, architecture, training, reviews, coding, testing, etc.

That's where you'd want a PM brought in, the tech lead ideally would work together with the PM on the roadmap, in that technology must be considered when designing a roadmap, feasibility, effort needed, technical complexities, needed tech dept pay backs, needed redesigns, infrastructure upgrades, security considerations, etc. As well as being consulted for what can tech realistically provide as means to solve problems, etc.

That's why I think a PM is needed, so the tech lead can focus on leading the technology, while the PM focuses on capturing requirements, getting funding, coordinating the launch, and all that.

Ideally, there's also a separate dev manager, so the tech lead doesn't have to worry about employee resourcing, project allocation, hiring, compensation, promotion, needs/wants, vacation time, performance, etc.

For all these things, the tech lead should be consulted and have a voice, but freeing them from all this work is definitely a plus, why have your most experienced developer do all this management work?

I think we'd call them PMs once the job gets big enough to need that work done (almost) full time.

Keeping up with the technical side, keeping up with the product side, and keeping the rest of the team informed is usually too much for one person.