Engagement sessions are important because the "need to practice" issue is hugely important for both me as the photographer AND the couple! The couple needs to get comfortable both with being photographed and with the photographer… and the photographer needs to get comfortable with the couple's personalities, styles, and quirks.
This is especially true if you or your partner is camera shy. Those of us who hate being photographed can get really awkward around cameras, and are prone to doing things like always making the same terrified/frozen smile every time the shutter snaps. A good photographer will work to get both of you comfortable, and look for ways to make you relax and settle in so that they can capture more natural, honest expressions during your special day.