A bicycle is a frame and a collection of stuff held together by different sorts of nuts, screws and bolts. Due to this fact, it’s very important to make sure that those nuts and bolts in question are tightened once in a while. Sure, you could carry a toolkit or always ride with a friend who has tools or just hope that nothing goes wrong and it’s those paranoid idiots who think their bike will break down during a 30mph descent.
Luckily, there are some engineers amongst bikers and they’ve watched too much MagGyver worked long and hard days to create the multitool. Which, astoundingly enough has multiple tools in one, and since it’s a bike multitool, those tools are usually related with your bike. They are usually also quite light in weight and small in size, weighing in at only a few hundred grams and nowadays they’re not even expensive anymore.
I personally use a Crank Brothers multi17 mini multitool, which has 17 different size tools including the basic screwdrivers, lots of different size hexa keys and even a chain tool (which I don’t know how to use, yet). Your multitool doesn’t have to be a brand version, just make sure it’s made out of proper material and feels sturdy in your hand before buying one, multitools under 10 euros are usually a bit wobbly or limited in their tool capacity, so make sure you check out different models before purchasing one.