I had to look it up. The word "spandrel" was originally an architectural term, then was adopted by biologists to describe just what the comic says.
Evolutionary biology uses the term spandrel for features of an organism arising as byproducts, rather than adaptations, that have no clear benefit for the organism's fitness and survival. In response to the position that spandrels are just small, unimportant byproducts, Gould and Lewontin argue that "we must not recognize that small means unimportant. Spandrels can be as prominent as primary adaptions". A main example used by Gould and Lewontin is the human brain. Many secondary processes and actions come in addition to the main functions of the human brain. These secondary processes and thoughts can eventually turn into an adaption or provide a fitness advantage to humans. Just because something is a secondary trait or byproduct of an adaption does not mean it has no use.
See, this couple could have used their spandrels better. They might have started working in the community garden only to impress each other, but if they had chosen to grow a vegetable that they'd actually want to eat, they would have learned to enjoy the activity. However, if they are happy just watching TV together for the rest of their lives, who are we to criticize? This comic is from Zach Weinersmith at Saturday Morning Breakfast Cereal.