My own guesswork answer: a 'useless project' necessarily combines a personal interest with the total absence of external obligations. Productive work sometimes

My own guesswork answer: a 'useless project' necessarily combines a personal interest with the total absence of external obligations. Productive work sometimes has the former property, but rarely has the latter. A successful useful unpaid project might feel like an obligation, if there are users and a community around it. Note: does not apply to Fabrice Bellard, with his remarkable talent for building incredibly useful software on a whim, on his own, in secret (at least initially).