You just never know when you are going to need this essential little bit of gig gear. I've been caught out too many times in the past. When you switch between different microphones you often need this little fella to adapt between the different mic stand thread sizes.
For example, I usually sing through a Sure SM58 (industry standard) microphone which fits into a mic holder. However, I sometimes use a Skull Head (Elvis mic) when doing a gig with a 50's rock'n'roll band. The Skull Head mic doesn't use a mic holder. It screws directly onto the mic stand. If I don't have the adapter, I'm screwed (forgive the pun).
I always have a spare adapter in my gig bag these days - I won't be caught out again.