The cycle length input is used by Glow’s algorithm to learn your baseline. However, this information is only used initially, and once you start to input actual cycle dates, Glow will use that data to predict your future periods.
Glow also doesn't use a straight average when predicting the cycle. The algorithm uses the average luteal + average follicular phase of your cycle history to make the next period prediction.
If you’re wondering how your initial cycle length stacks up with your averages in Glow, go to Home > Calendar > List view. There, you’ll see the cycle lengths that you’ve previously reflected in your start and end date inputs.