Or to scratch their "do it right" urge, to have a repo that they can show potential employers, to learn TDD, to learn a new language they can't use at work but

Or to scratch their "do it right" urge, to have a repo that they can show potential employers, to learn TDD, to learn a new language they can't use at work but hope to use professionally at some point, to have a repo that they can eventually turn into something revenue generating, or because they contribute to open source projects that require it. I've written tests on personal projects for all of those reasons at one point or another (except the last).