It's mechanical impact is that your hands are free while carrying all your crap.
A sack is for carrying your food rations. As in..
Sack... food.
Waterskin.... water
Quiver.... arrows
It says that your starting gear includes a knapsack,
Page 36: "STARTING GEAR: Your profession determines what gear you can choose from at the start of the game. As a rule, you get one or more items from the list of trade goods on page 182 (no
RARE items however). If it says, “a weapon of your choice,” pick one from the weapon tables in Chapter 5. You cannot choose a magical artifact.
In addition to these items, you are assumed to have a knapsack and a waterskin. Gear used to carry other gear does not count toward your encumbrance and does not need to be noted down."
So we houserule that the Backpack gives you a couple of extra inventory rows, as a good backpack allows you to carry more then a just a sack over your shoulder.