Use to glue acrylic sheet together

Generally glass bonding is done with a special adhesive that is UV cured. I'm uncertain as much as far as the cost for it. I have always used silicone to bond glass, it's what my family business uses to assemble/repair fish tanks.

The silicone would also work perfect for you in place of grout.

It is almost impossible to know their original intended usage. Perhaps they were "windows" in a set for some old play, then removed and saved after the set was torn down.

My guess is the person scavenged then from some application like that, then kept them hidden away in case they ever found a new use. That you found them still hidden says they never did find a use, and that you won't either. However, I'll bet that you can sell them on Craigslist or ebay. Or bring them to your local auction house, where all sorts of things like this get sold.