I've worked jobs by the hour. I've also been self employed so I worked for the work. I like the self employed better so it means that every minute matters, as opposed to working by the hour where every minute was 1/60th of the pay rate.
That is like field service. Working by the hour "I'm going out for 3 hours today", you focus on the 3 hours and watch the clock. If your ministry is not about time, then you are going to focus on people not the clock.