Why does Super Walmart and a few other stores have so many check out lanes but only use very few?

Seems like they only use ten or less, plus the self checkout.

Are they understaffed? Does the store just not want to pay their employees to run them? And the store could be packed with customers but the remaining registers will still be closed. Once in a blue moon they might open one to move things faster.
Why did they build all those checkout lanes if they never use them?