Eons ago--seriously, in the dark ages folks--I read Battlefield Earth. It's a huge book, and while I sort of enjoyed it at the time, nothing sticks with me except really tall aliens and that L. Ron Hubbard was very verbose.
Except for the concept of rolling in a plot.
That was the first book where I became conscious of the ups and downs in storytelling. At the time I thought of it like a roller coaster. Or what I call down time. We've all read books where things just keep building and building with no let up, no quiet moments. They can be exhausting! (Some folks love that--more power to you!)
For me as a reader, a book needs to have an ebb and flow as it works its way up to the "Everything has gone to hell and we're all gonna die" moment. I like those quiet moments in a book. I can breathe a bit (a good writer will have you racing alongside the characters), there's often a bit of humor, or character development that makes the characters a little more real. Then I'm ready when the next massive battle, attack, mission, or whatever, happens.
This is more noticeable for action heavy books. For me, if there's too much going on and not enough down time hanging with my peeps (aka book characters ;)) I usually don't enjoy the book. I'm all for a great plot, but I LOVE characters I can fall in love with.
But even more subtle stories can benefit from the roller coaster. They too, can benefit from stepping out of the story for a moment. The sitting around a kitchen, the club, the favorite diner. Just a chance for the characters to decompress and show sides that might not have been noticeable before.
But please, don't just use that time for an info dump or recap of what just happened in the story ;).