Assume you go out to lunch four days per week. Each time, you pay for your own $7 lunch. Four payments of $7 = $28.
Now, assume you buy that same $7 lunch for you and your colleague twice per week. And, they return the favor. Two payments of $14 = $28.
Same cost. But isn’t it nice to treat someone? And even nicer to not bring out your wallet twice per week?