I believe most of those who use this phrase fail to understand the context of that phrase. Below is my own paraphrase of 1 Cor 14:13-19 which might make the meaning more clear. I encourage you to get your favorite version of the Bible and read this passage for yourself and see if my paraphrase hits the mark or not.
Therefore, one who speaks with the spirit, that is, in a tongue, should pray that he may interpret.
In other words, if I pray in the spirit, I will also pray with the understanding. If I only pray in the spirit without praying with the understanding, then my spirit does just fine, but my mind is left empty. That's why we need an interpreter.
Not only praying, but singing; if I sing with the spirit, where no one, not even myself, understands what I'm singing, I need to pray to interpret so that I can also sing with the understanding. Otherwise, if you give thanks just with the non-understandable spirit, how can another person understand your giving of thanks? If he says "Amen" to your thanksgiving, he doesn't even know to what he's agreeing. And you may be giving thanks just fine as far as God is concerned, but the other person gets nothing out of it.
I'm thankful that I speak in tongues more than all of you. Still, in the assembly, I'd rather speak 5 words that are understandable to everyone than 10,000 that do no good to anyone except perhaps myself and God.