It is undoubtedly useful.
Every time a client chooses this option, Upwork earns additional money without any real additional expenditure of resources.
Whether it would be useful to you as a client may depend on the type of job you are posting, as well as your skill at creating job postings.
Personally, I don't find that I need it.
I can post free job postings for very specific, very technical problems, and then get excellent freelancers who apply to my jobs within ten minutes.
I routinely post jobs and hire freelancers and then take the job offline because I found someone great all within ten to fifteen minutes.
I have been very pleased with how easy it is to post jobs and find excellent artists and graphic designers. All without paying anything for job posting.
I guess I am the wrong person to say whether it would be helpful to you or not. Have your regular job postings been unsuccessful in helping you find people to hire?
The more the job is worth, the more the "featured" feature is worth.... Why?
Because top rated freelancers pay a reduced (half) Upwork fee on the first $ 500. So to me, on a $ 500 or $ 2000 contract, that means an extra $ 50 in my pocket. That $ 50 extra motivates me to apply over an equally attractive job post that is not featured. Because $ 50 is $ 50, right?
If your job posts are typically lower budget, or aimed at freelancers with lower rates, and if you are not so fussed about whether you attract top rated freelancers or not, it probably doesn't benefit you that much.
The same is true if you are hiring in categories where you are typically already swamped with proposals.
Even through Upwork does not specify the number, they do verify here, https://support.upwork.com/hc/en-us/articles/115010165067-Featured-Jobs, the basics of what you say: "Freelancers and agencies receive a reduced freelancer fee on your project"