Why are PID parameters set with relay output?

It's time proportional PID. That means that in addition to relay output, it can support TRIAC and voltage output. With time proportional output, control calculation result is output using the pulse width of an ON/OFF signal proportional to time. The pulse width is calculated by the following formula with the cycle time (control output period) being 100%.
Control output pulse width = control output (%) x cycle time