4mm, 5mm, and 6mm bolt heads are the most common sizes on your bike. 90% of water bottle boss bolts use an M5 bolt, which takes a 4mm hex key. Lots of stems, almost all chainring bolts, many seat posts and so on use a 5mm allen key. We sell these because everyone needs a few of these keys around. We have extras laying around the shop. You kinda always need one in reach whence working on your bike. However, the real place these keys come out to shine is roadside bike repair. We love a good multi tool, but for some hard to reach places, a loose hex key is the ticket to a quick and sweat free repair. Water bottle cages and most multitools hate each other. Weird fender bolts by the bottom bracket do too. Even lots of seat posts can not use a multi tool to be adjusted. Critical, when you need to make an adjustment though!
Bonhus makes all of their tools in the US, in the Midwest, so you’re guaranteed that your tool has a droll sense of humor and makes a mean old fashioned.