serve (someone's) purpose
To fit or satisfy someone's requirements; to be useful for or fit to achieve someone's aim, goal, or purpose. Well, it isn't a very pretty car, but it should serve our purpose just fine. Child: "But mom, I want a smartphone!" Mother: "Nonsense, the cell phone you already have serves your purpose just fine."
serve the purpose
To fit or satisfy the necessary requirements; to be useful for or fit to achieve some aim, goal, or purpose. Well, it isn't a very pretty car, but it should serve the purposes of our mission. Child: "But mom, I want a smartphone!" Mother: "Nonsense, the cell phone you have serves the purpose just fine."
serve (one) the same sauce
To treat one the same way that one has treated you—usually poorly. Of course she's not invited to the dinner party! She didn't invite me to her last soiree, so I'm serving her the same sauce.
serve two masters
To simultaneously tend to or support or devote oneself to two different—often conflicting—responsibilities, pursuits, ideas, or people. It comes from the Biblical phrase, "No man can serve two masters." You need to decide if you are married to your wife or to your work because you simply cannot serve two masters.