Singers sometimes get a little caught up in the music (or themselves) and move around when they sing. You've probably seen the cantor or lone chorister swaying their torso, taking a huge, obvious breath or move their chin up and down based on the pitch. All these things get in the way of good singing and music-making.
Moving too much creates your own rhythm. If you get to swaying, bobbing or tapping, changes are your tempo will be different than the conductor. Don't externalize rhythm - watch the conductor and follow his rhythm.
Moving too much adversely effects your instrument. There's a reason organ pipes are a fixed size - the size and characteristics dictate the pitch and timbre. Same thing with the voice - you can't have a consistent, good sound you are moving your instrument around. The entire instrument, from air in the lungs to the shape of the throat and other areas where there's resonance shouldn't be moved around.
Moving around is distracting. There's no benefit, particularly in a liturgical setting, to drawing attention to yourself in such a silly way. Someone might give you a leotard and a big, curly banner and ask you to lead the liturgical dance ministry.
Moving around means you're probably not paying attention. If you are busy "emoting" you are probably not engaged with the rest of the ensemble.
That covers it. If you stay still, you'll look better, sing better and be a more valuable part of the ensemble.