First of all, not all of them have.
Rim brakes, which is the other option, have a few drawbacks. In rain, brake performance gets worse. If you ride in mud the brakes will eventually wear the rim away. If your wheel gets warped brakes won't work that well.
Using disc brakes, which sits in the center of the wheel gets you away from all that.
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Some Freestyle and street BMXers ride brakeless. It's all up to you and your comforts.
BMX bikes typically have one brake, in the rear. A rear brake is enough to slow a BMX bike or eventually stop it.
If you do all your riding in a controlled environment like a skate park, then there isn't much use for a brake, and it's one more thing that can get damaged when the bike is tossed around. But if you ever ride in traffic it's very sensible to have at least one brake. Usually legally required too.
Bikes have their pegs on the opposite side of the bikes drive.
Right Hand Drive (RHD) will have pegs on the left side,Left Hand Drive (LHD) will have pegs on the right side. This makes it easier to grind.
Any bike that is ever ridden in traffic should have at least one brake, and is usually legally required to do so.
Apart from that first there are track bikes. They are intended to be ridden on special race tracks only with everybody going in the same direction, and in a close bunch. Not many sources for surprises (as out in traffic), and when you're basically riding shoulder-to-shoulder there's not enough time to put on the brakes if something should happen either.
Then there are the BMX style bikes for jumps and tricks. These aren't meant to be ridden in traffic either, and brakes to them would just mean another thing that can break when the rider falls or have to bail out.
Then there are those who think that either of the bike types above look "cool" and decide to ride one in traffic anyhow, creating a danger not only to him/herself but also to anyone else (s) he might encounter.
And there are those who'll ride their brakeless bike to/from the track or bike park, just b/c they have no other way of getting there.
Mostly for fashion.
And if you ever plan to ride in traffic, brakes are generally both legally required and very useful.
Although for some kinds of trick riding, brakes aren't very useful and mainly becomes one more thing that can get busted when you crash.
yes you can have one brake . some people have a back and front and some people ride with none like me . its all down to you personal likings.