Why does Observable.interval() operator uses the computation as default scheduler?

Question

In my Android app I'm using the interval() operator to schedule multiple periodic jobs (which may run simultaneously):

public class Job {

    ...

    public void start(Runnable runnable, Scheduler scheduler) {
        Observable
            .interval(this.initialDelay, this.interval, this.timeUnit)
            .observeOn(scheduler)
            .subscribe(t -> runnable.run());
    }

    ...

}

My questions is:

Why does it use the computation() as default scheduler for the interval() operator?

obviously I can use the overloaded method and send different scheduler but I'm not sure how it may affect the performances or anything else by changing the scheduler.