Folding bikes are generally sold as one bike fits most. This is possible only with a stem that is height adjustable. Aggressive riders will prefer a stem that is low so that they will be able to get into a tuck position. Recreational riders will prefer a taller stem which will provide a comfortable vertical sieated posture.

Overall everyone is different, hence allowing for stem height adjustment will help accommodate a wider range of body structures. Don't buy a folding bike if the stem is not adjustable....the comfort will be limited.