The useful idea
Buy a project when the result can be bounded and you want a partner to own delivery coordination.
When a project fits
You can describe an outcome: move an application, make releases repeatable or connect a recurring workflow. You want a delivery partner to organize the technical work and demonstrate completion.
When staffing fits
You need ongoing capacity inside your own team, and your managers will direct daily work. Hiring or staff augmentation may be the right choice.
What still stays with you
A company delivering a project still needs your requirements, access, timely decisions and acceptance. Agree those responsibilities before the work starts.