Jussie, I start out trying to use what I have to complete an outfit, but sometimes I say to myself, "What this outfit really needs is X!" And then I will go out searching for that thing. An example is this pink scarf from Nordstrom, which I bought to match some light pink shoes. I didn't have anything else pink in my wardrobe, so I had to get something to match the shoes (I like to repeat colors in an outfit).
On the other hand, certain items that I think are completers were bought at random...such as my Madewell sweater dress. It is not so bold as to be a statement, but it's not an essential either. That makes it a completer. I like to wear it with black faux leather leggings, which I also consider a completer. I did, however, buy the leather leggings specifically to wear with the dress.