What are those bungee cords on (backpacking) backpacks for?

bbkf101

New member
I've just purchased a 50lt backpack for a trip around Europe. It's got these annoying bungee cords across the top flap and some in the middle of the pack as well. What are they for? Please suggest some uses? Are they really necessary? I'm tempted to take them out. Pardon my ignorance but please enlighten me.

Thank you in advance. Cheers.
 
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