Not just spare capacity for emergencies. Think about how many times an engineer came up with a good idea how to make some part of the process smoother, faster,

Not just spare capacity for emergencies. Think about how many times an engineer came up with a good idea how to make some part of the process smoother, faster, less error-prone. It's usually in some phase of the project where there's not too much to do or deliver. But these small gains are necessary for everything to work well then the pressure is on a month or two later. You don't get this if you're always at capacity and have no time to think.