There's a practical reason for it!
Do you see those green flecks of goodness in the photo above? Celery may be your chicken salad's best friend, but there's good reason you should delay adding the fibrous veggie until right before you're ready to dig in.
"Once it's cut, this veggie releases liquid, which may cause your [chicken salad] to turn into a watery mess," explains the Good Housekeeping Test Kitchen staff. "To prevent this, mix in the celery right before you serve. Otherwise, simply add another spoonful of mayo or yogurt." Since adding more mayo or yogurt can change the flavor (and calorie count!) of your chicken salad recipe, we'd advise saving the addition of celery until the last possible moment before mealtime. If you forget and add your celery prematurely, or are using leftover chicken salad, this technique should work just fine, though.
WATCH: Chicken Salad with Grapes and Pecans
Of course, some people don't care for the green stalks, so you may consider leaving celery out entirely. Then again, mamaw's wrath for tinkering with her generations-old family recipe may not be worth taking that risk.