Seth Godin says

The magic of slack (a little extra time in the chain, a few extra dollars in the bank) is that it gives you the resources to stop and avoid a problem or fix it when it's small. The over-optimized organization misunderstands the value of slack, so it always waits until something is a screaming emergency, because it doesn't think it has a moment to spare. Expensive.

Slack also comes in handy as a way to encourage the new. The first time you try cooking at home instead of ordering takeout it will cost more--spice jars are expensive!--and take more time. Trying a new route to work, folding your laundry in a new way, hiring someone to do a task you currently do, these all need slack.