What is purpose of these holes in shower enclosure?

[Old UKPF 133282]

I have holes in the wall of my shower enclosure. Down low at the open corner, between the quartz wall and the solid pan. Same at each end of the shower. They are obviously intentional.

But they seem stupid to me. Just a hole where water can intrude thru the quartz wall to whatever is behind. Wtf?

Should I seal them up? What purpose do they serve?

I think I attached pics.

 

[oz-plumber]

Looks like something cast in the bath for overflow provision.

Doubt it goes anywhere, unless you have a drain connected to it behind the wall.
Not sure how you would check it though.
In the first pic it looks like there is silicon in there anyway.

Check and if so silicone it up