For a variable depth bridge deck, the depth of continuous multi-span bridge deck is increased in pier supports and this absorbs sagging moments in the mid-span with the consequent increase in hogging moments in pier supports.
As a result, the mid-span depth can be significantly reduced due to the reduction in sagging moment. In essence, this sucker deck principle is applied in locations where headroom requirement is of great concern. Moreover, in terms of structural performance, sucker decks are effective in reducing dead loads than voided slab of equivalent uniform depth for span length between 20-40m. In terms of aesthetics point of view, the public tends to appreciate the structural form of arches and curved soffit rather than boring uniform deck alignment.
Ref: Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.