Would you rather wait 45 minutes today or not be able to see your doctor til next tuesday? Because those are your options with services that have unknown lengths (see also, cable/satellite installers). You either overbook the person knowing they'll have too many appointments, but will constantly be working, or you underbook them so you are positive every single time they will be done with the previous appointment.
When you underbook them, they don't know whether the job is gonna be a quick "oh you have a cold, here take this prescription and you can go 5 minutes later, or a really long hour long exam thats needed, so that full hour is booked, that means if you DO come in with that 5 minute cold, they are sitting there for 55 minutes doing nothing, because their next appointment isnt for an hour. This means they don't see nearly as many clients in a day which means instead of being able to see that sick person today, they cant do it til next week.
This is why they schedule people in 15 or 30 minute blocks, knowing full well the likelyhood they will be seeing you exactly 15 or 30 minutes is next to 0. You will either be much less, in which case they can go right on to the next person in line, or you'll be a bit over, in which case the next few people in line will probably have to wait a while. They catch up with those 5 minute cases or when people dont show up. This way they are working and seeing people the entire day.
When you underbook them, they don't know whether the job is gonna be a quick "oh you have a cold, here take this prescription and you can go 5 minutes later, or a really long hour long exam thats needed, so that full hour is booked, that means if you DO come in with that 5 minute cold, they are sitting there for 55 minutes doing nothing, because their next appointment isnt for an hour. This means they don't see nearly as many clients in a day which means instead of being able to see that sick person today, they cant do it til next week.