Semi-trailers

A semitrailer is always Coupling with truck tractor for cargo, vehicle, bulk, fuel, water and others transport service.
Semi trailer is Semi truck without front axle, so some weight is supported by coupled truck tractor.
Semi trailer can be simply coupled and uncoupled with tractor, and one tractor can drual several semitrails. If semi trailer is broken, it's easy to change another one.
Compared with full trailer, it's easier to attach semi-trailer to tractor, and simple to control semi trailer turning during driving too.