One reason to lower the water level of a system you count on is to see what's messing things up. You can discover what happens when you operate without slack, without a surplus... you want to know what's likely to get in the way...
This is the essence of Toyota's quality breakthrough. When Toyota got rid of all the extra car parts held in reserve on the assembly line, every single one of them had to be perfect. If a nut or bolt didn't fit, the entire line stopped. No cars got made until the part was perfect.
This seems insane. Why would you go through the pain of removing the (relatively) low cost buffer of some extra parts? The answer, it turns out, is that without a buffer, you've lowered the water level and you can see the rocks below. Without a buffer, every supplier had to dramatically up his game. Suddenly, the quality of parts went way up, which, of course, makes the assembly line go faster and every car ends up working better as well.
Fedex had to build a system far more efficient than the one they use at the Post Office. When you only have 12 hours to deliver a package, the rocks will kill you. Now, when they need to deliver something in three days, they're still way better at it than the post office is. Fewer rocks.
The purpose of sprinting without slack isn't that you will always be sprinting, always without extra resources or a net. No, the purpose is to show you where the rocks are, to discover the cruft you can clean out. Then, sure, go back and add some surplus and resilience.