There are a few advantages apart from straight up hourly rate:
1) The big one is that you get a ready-made team of project managers, UX designers, QA, developers that have either worked together before or at least share methodology and culture, and so have already gelled somewhat.
2) People in these various roles come in when you need them, and leave when you don't. You might not be able to afford to hire an experienced designer or QA full-time, but you could afford a couple weeks of their time, or two days a week, or whatever. This flexibility can make the end result higher quality and less expensive, even with a higher per-day rate.
3) It's hard (some say impossible, but I wouldn't go that far) for non-technical people to hire technical folk, because it's hard to know what to look for. A consulting firm has already done this for you, has a track record with these people, and is staking their reputation (and hopes of future business with you and your contacts) on them.
1) The big one is that you get a ready-made team of project managers, UX designers, QA, developers that have either worked together before or at least share methodology and culture, and so have already gelled somewhat.
2) People in these various roles come in when you need them, and leave when you don't. You might not be able to afford to hire an experienced designer or QA full-time, but you could afford a couple weeks of their time, or two days a week, or whatever. This flexibility can make the end result higher quality and less expensive, even with a higher per-day rate.
3) It's hard (some say impossible, but I wouldn't go that far) for non-technical people to hire technical folk, because it's hard to know what to look for. A consulting firm has already done this for you, has a track record with these people, and is staking their reputation (and hopes of future business with you and your contacts) on them.