I am not a criminal barrister, so if one happens to come along (or a criminal solicitor, for that matter) they would be better placed to answer this. However, my understanding is that duty slots are very much seen as loss leaders. So they're not something that sets or individual barristers do to make money (as you've rightly noted), but they are something that can lead to an increase in work, either for individual barristers or for the set generally. It is worth noting that things like this, namely doing lower paid in the hope of receiving better quality and better paying instructions further down the line, exist in lots of different areas of practice. It may be individual cases, such as a solicitor wanting a barrister to accept a lower fee than normal on one particular occasion, or it may be subject to an agreement with a particular firm or insurer. Sets may agree to handle insurance backed work for firms on low fixed fees, for example, on the expectation that they will also receive instructions on the better paid private paying work from the same firm. My understanding is that duty work is similar. I don't think it's right to say that it will lead to an increase in work, let alone a substantial one. But that is the idea behind it. Like every similar business decision, I expect it's something that is subject to review and part of the overall strategy put in place by clerks and more senior members.