What is a truck-tractor primarily designed for?

A truck-tractor is primarily designed to draw other vehicles, which is the function that distinguishes it from other types of trucks. Typically, a truck-tractor is equipped with a powerful engine and chassis that allows it to pull a semi-trailer or a series of trailers, often used in long-haul transportation. This combination enables the truck-tractor to effectively transport goods across significant distances while maintaining stability and control.

In contrast, options such as carrying heavy loads focus more on the cargo capacity of different types of vehicles, which is not specific to truck-tractors alone as other types of trucks also perform this function. Transporting passengers would be characteristic of vehicles like buses, which have a completely different design and purpose compared to truck-tractors. Servicing other vehicles is a role typically associated with service trucks or maintenance vehicles, not truck-tractors, which do not have the specialized equipment required for such tasks. Therefore, the primary design purpose of a truck-tractor is best captured by its capability to draw other vehicles.