For incremental launching method, the span depth ratio of bridges is normally low. Why?

Bridges constructed by incremental launching method are usually low in span depth ratio and typical values are 14 to17. With low span depth ratio, the bridge segments are stiff in bending and torsion which is essential to cater for the launching process. Such low span depth ratio could tolerate the discrepancy in vertical alignment on supports over which they slide. Such differential settlements may occur owing to the shortening of piers when the superstructure slides over them and the differential deformation of different piers.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.