drainage條 margin冇左會唔會更好？

If adjacent to the drainage is concrete slab,  what is the purpose of the drainage margin?

While,  normally,  it would reduce the support width of drainage cover.

Would it be possible to make the adjacent concrete slab (say 100 width)  50mm thicker instead,  then,  it would keep the abate support width. 

如果adjacent 係路， 會唔會冇左條margin 更好？ 或者條margin應該做出渠邊， 即2旁的路做厚小小。