There is a wide range of different loops on the outside of a backpack that are there to carry a variety of equipment, hold items in place or connect to other pieces of luggage.
Most backpacks have side compression straps on the right and left sides, which can be used either to compress the volume of the bag or to connect bulky items like tents or shoes to it.
Shoulder strap loops are used to attach small items like water bottles, phones or GPS systems to the shoulder straps of your rucksack for easy access. Similarly, hip belt loops are there to carry small items at your waist.
Most larger backpacks have rear loops at the bottom of the bag which can be used to attach sleeping bags or camping mats to the rucksack.