It's one of the skills that indicates someone is more than a software-engineering, misanrthopic robot. This world has enough of those.

Signalling is an important thing, it's why you iron your shirt before the interview - it may even be why you have a degree. The cost of false positives is high, so if this filters out skilled engineers who think are appeasing the interviewer's ego, the system is working.

If you can’t show me you’re a senior+ engineer, then I’m not going to hire you for a senior+ job. It’s as simple as that. You can ace the technical interview, but if you can’t tell me why I should choose YOU over every other candidate that aces the technical interview… why shouldn’t I pick one of those other ones? It has nothing to do with ego or anything else.

This is a skill, learning to sell yourself. It translates into being able to get sell innovative projects to stakeholders, to learning new things, to understanding business requirements.

Maybe I’m wrong, but I’d love to hear why or how.

Yes, I am completely with you. I always hated these arbitrary algorithm tasks that have nothing to do with the actual job. With the experience of hosting multiple job interviews I prefer to do a task that is close to the job, but keep it open enough for the applicant to do it their way. And making sure to the applicant that asking questions is desired. If they go in a wrong direction without asking questions, it's a good sign. If they ask good questions it's a strong plus point. Also I can see if they can apply learned knowledge quickly.

Why should I do algorithm or math related tasks as a frontend developer for example. Soft skills are way more important than preparing for potential interview questions.

> throw away lots of good candidates.

You say this as if it's a bad thing.

The reality is, companies have multiple qualified applicants for each open role. Hiring teams are actively raising the bar and trying to find more ways to narrow the pool down. If there are 10-20 qualified applicants in the pipeline, and half of them interviewed terribly, it's very easy to cut the applicant pool in half by rejecting the people who couldn't communicate their experience clearly during the interview.

Remember, "interviewing skill" is just another way of saying "how good are you at communicating with prospective employers".

Communication is usually an important hiring criteria. Ability to communicate well in an interview ("good interviewing skills") is a good proxy for ability to communicate in general.

To play devils advocate interviewing isn’t far from presenting your ideas to a skeptical audience. This is a good skill to have for any engineer. I don’t think it’s necessary or sufficient but it is useful.