Don't know the ideal scenario in the tech industry but in the advertising/marketing industry, it's useful to charge by a project.
One may try to calculate (in their mind) how much time they will take to complete the project and then quote a fixed amount based on the no. of hours that came up.
I've made movies and software. I'm extrapolating from movie experience to imagine I know something about marketing and advertising.
I'd feel more comfortable presenting a fixed project quote for a creative project than a software project, because in the case of a creative project I can choose to dedicate a fixed amount of time, and have the result - to be judged subjectively - that I was able to produce in that time.
In the case of software, I would need to keep working until it objectively meets requirements. The quality of software I deliver doesn't degrade smoothly under time pressure, it falls off a cliff.
an essential aspect of fixed hour contracts is that requirements need to be sorted from the most critical to the nice to have and the client needs to be prepared to drop the nice to have if the budget doesn't stretch to them
which is the whole point of the agile manifesto
which is why agile fails under most circumstances, as having fixed time, requirement and budget but under scrum and sprints is anything but agile.
One difference that comes to mind is that while in advertising / marketing you'd need to get what your clients offer, what they'd like to be offering, and how to work with them, in tech you'd also need to understand how it is currently done. The fact is, it's often a mess, with lost, fragmented and conflicting knowledge.
And while both need to deal with unclear goals, figuring out what it is the client and truly needs, in one case you're selling what they have, or what they are, in the other you have to make something they want and will be able to use, support and sell.
That's very often a tremendous amount of uncertainty to work through.
My work has mainly been with small businesses on limited budgets, so while an hourly rate might be agreed, I'll also agree to be realistic about limiting costs. Here communication and trust are key, we'll discuss core essentials, nice to have and blue sky dreams, we agree to a fixed limit on costs, I start the project, then, once I'm 20% or so into the budget I'll go back with a clearer picture of the project and the client can decide to spend more, or cut back on non-essentials, at this point the level of trust is high, they know I am keen to deliver good quality work while respecting that the cost needs to reflect the benefits and the mutual understanding of what we our aiming to achieve has been reinforced.
One may try to calculate (in their mind) how much time they will take to complete the project and then quote a fixed amount based on the no. of hours that came up.