Just one drop causes the bucket to overflow. It's a well-known saying. Still, you don't want to experience it while brewing. That's why an extra bucket is always handy, e.g. as a fermenter for those who work with a starter kit. It's especially useful if you want to go bigger (read: as you get thirstier), as you'll quickly need extra capacity as you complete the various steps – whether you’re making beer, wine, cider, juice or kombucha.