This may be a noob question. I am sorry if it is but I am new to this field. I am working on a design of SCARA Robot and I am using Stepper Motors to drive it. In many youtube videos, I have seen that they increase the torque using timing belts. Now I understand that in most cases timing belts are used for keeping motors away from the joints to reduce inertia. But I don't understand why they use ratio in belt drive. As far as I know, Stepper motors provide high torque at low speeds and vice versa. Then Stepper Motor should provide that much torque at reduced speed by default, right? Or I am missing something?
Even at zero speed, stepper motors have a max torque, called the holding torque. If your design needs even higher torque than that, you could use a bigger (heavier, more expensive) motor. Alternatively, use a ratio in the belt drive.
To be concrete: a 1:2 ratio will double the available torque, but at the expense of making things 2x slower. It will also double the resolution by making each stepper step half-size.
A design with a very high ratio would start looking like a gearbox, which would suggest revisiting the approach.
As r-bryan answered they do increase the effective torque. However that also increases the effective resolution giving you better repeatability. As an example a direct drive motor that provided a 0.1 degree repeatability using a 4 to 1 ratio belt drive would give you a 0.025 degree resolution. The trade off is a lower speed.
As secondary effect is that in case of a collision, the belt will absorb a certain amount of the impact possibly preventing damage to the motor.