Many television shows--Jags, Bones, Friends...too many to count--will placate viewers by giving them a secondary couple whilst the viewers wait for the primary couple to work things out. Occasionally the secondary couple becomes a bigger deal than the primary couple (Monica and Chandler, for instance). But quite often, they are simple a break on the way to the primary couple's consummation/marriage, etc.
Even when the secondary couples are just a blip on the radar, they are often more stable and consequently, in the long run, more believable than the primary couple. Bud and Harriet from Jag are a good example. Sarah McKenzie and Harm are (supposedly) the primary couple, but I'm not sure that they ever move forward in a believable fashion. Bud and Harriet, on the other hand, are practical, kindly and straightforward; they genuinely like and love each other and have similar life goals.
Major Crimes is another impressive exception. Although Brenda and Fritz's growing primary relationship runs The Closer (and is quite well-written), the primary relationship of Major Crimes--Captain Raydor and Flynn--is accompanied by Patrice's marriage to Provenza as well as Rusty and Gus's relationship, which is not abandoned, as I assumed it would be, at the end of Season 5.
And then there is the relationship that seems secondary but is actually primary: Niles & Daphne. Yup, watch the show enough and it becomes clear that that relationship was being set-up almost from day one (Niles's infactuation wasn't simply fodder for comedy): but then, these are writers who will establish one small event in minute 1 of an episode, only to pay it off in minute 20.