> For the same reason artists do
They're asking more how it would happen, less for what purpose.
Artists automatically accumulate portfolios by doing work that gets displayed in public. Programmers who work on closed-source programs don't—unless they supplement their work-for-pay in ways that artists aren't expected to. The artist's history of work itself is how they get more work; the programmer's isn't.
open source projects in a relevant industry. IME it's never been required to have sample work to show off, but many agree that having some OS contributions or even small pet projects is one of the most valuable things to leverage, outside of actual work experience.
>The artist's history of work itself is how they get more work; the programmer's isn't.
No one is really going to care behind closed doors, but _technically_ there's plenty of concept/scrapped art owned by the company that they can't directly point to on their public portfolio. It's not quite as easy for artists either to show off what may have been their best work.
> The artist's history of work itself is how they get more work; the programmer's isn't.
It's just a recommended strategy, not a requirement. In general, it's a great idea to identify a customer's pain points and reduce them. When you're looking for a job, the companies you're talking to are the customers. The easier you make things for the interviewer, the more you can reduce their pain points, the better your chances for the position.
If you're trying to explain how they can do it with the least amount of extra work, to minimize the impact of the extra expectation, please consider that not everyone has hobby projects. Even fewer people's hobby projects are presentable, and even fewer still can trust the other person not to draw some wild conclusion based on anything that might be in there.
I think everyone is clear on how the situation works, but not everyone is happy with it working that way.