Here is my opinion, only because I just posted a question about a control valve box: besides being able to use your finger to open, this hole could be a way for excess water to escape.

One of my boxes keeps filling up with water so this hole could be for overflow?? Again, my opinion only!

GW,

Same reason there are holes in a manhole cover. A lever is used in the manhole lid (usually a pick or crow bar). The valve box lid is similar only on a smaller scale. It is for inserting a lever (usually a screwdriver or valve rod) to open the cover when dirt is compacted in the lid edge and the lid is almost impossible to open with a finger. JMHO ALoha