system
Given the following post, respond as if you're commenting on Reddit:
human
What purpose do the ridges & depressions common in food containers serve?

They are present on the lids and sides, make manufacturing more difficult, and use more plastic rather than a flatter surface.

Initial thoughts are maybe to disperse heat better or maybe lower surface tension (making it less susceptible to cracking)

Edit: Thank you to everyone for your replies! 
For future readers the answer is essentially one word: corrugation. Corrugation adds strength and durability while using significantly less plastic than would be necessary for a flat border with similar durability &amp; strength. Another benefit is grip (think bottom of a bottle wouldn’t stay put as easy if it had a flat bottom) and some ridges are potentially remnants of the manufacturing process (unconfirmed).
gpt
They provide structural support.  If everything was flat plastic it would bend so much it would either crack, or be so flimsy as to act as a stiff plastic bag.  It actually saves plastic because the container would have to be a lot thicker otherwise.

Look at the difference between bottles with flat sides vs something like a Gatorade bottle.