Allowing substitutes allows someone to fill in for your regular Pro if they can't make it. If you only allow your preferred Pro to come, then you can manually request one later, but it is less likely to be successful (because when finding a substitute you want to start the search ASAP). For example, let's say you have a 8am job with Maria and she gets sick. If you don't allow substitutes, you would have to get the job with Maria on a different day when she is better. If you did allow substitutes, someone else could fill in for her.
This also allows us to authorize Standbys for a job. Standbys are pros who are in the area, knowing you may need them. In the event they are needed, they are able to get there quickly.
Most people, for most cases, don't have "one exact time or not at all". By setting backup times, you still have a confirmed timeslot, but in the event some flexibility is really needed, then the job can still happen. For example: Let's say you have an 8am job, and the Pro has a car accident and can't make it at 7:45am. Without backup times, you would be unlikely to get a substitute to come within 15 minutes. If you set a backup time for 9:30am, you would have plenty of time to get a substitute.
This way, if there is any issue with the job, a Pro has the time to come make it right later that day (or the following day). For example: if you have a party at 8pm and you schedule a 2.5 hour job for 5pm. Even if you get 2 Pros, if they shut down the freeway at 4:30pm, they may end up too late to help as much as you'd like. By scheduling early in the day, you enable more opportunities to respond to issues. For example, if you had the job done at 8am, but something similar happened, you could get it done later in the day.
If you have another vendor, even one who is not your favorite, add them. TIDY will reach out to other Pros you add to your list when issues occur in your priority order.