When set up, Service Resources will also reflect on to your Online Booking Calendar, you may see that you open gaps on your Calendar but if they conflict with Service Resources with your other bookings these gaps will disappear when the Client tries to Book Online.
Without Service Resources the Calendar may be open and allow the Client to book at Anytime.
Then after you set up Service Resources this will limit the Client to only book when the Service Resource is Available
For more info on Bundles, be sure to read the Service Resources Overview.