It is a requirement for all of the military projects I’ve worked on. Specifically ACAS which has Nessus as a component.
For a solo person doing scans for clients I think the cost is like $2k. Seems fine but I wouldn’t buy it without a guarantee I’d get my money out of it.
There is one other product I’ve tried from greenbone. Never used it professionally. Idk if I would trust it at the same level as Nessus.