Hi,
You can test Port B from your main program loop anytime you want, however you may find that restrictive if you are doing other intensive work in the main loop.
Using the interrupts relieves that Port testing pressure from the main program loop.
Typical example would be a door switch connected to RBO so it would Interrupt the main program loop when activated, the main loop does not constantly have to test RB0.
Many timers use the Interrupts, the main program loop sets of the Timer , when its short delay is complete it sets its Interrupt flag On, the ISR is activated where the timer is Reset, re-activated and user registers / counters are updated without the main program loop doing any work.