Should I fill the hole under this window with expanding foam?

[vagrantly3893]

I took off the window sill and saw there was an area of the window that does not have any expanding foam. On the other window, there is expanding foam all the way along the window.

Is it possible this gap was left for a reason, or is it recommended to fill it?

 

[Bonner]

No reason for the gap, fill it up!