I see these things at construction sites pretty often and have no idea what they are for.
It’s about 5 feet tall, 2 feet wide, 2 feet deep, 3-inch wall thickness and hollow. Big circular indents on each side. They’re usually empty, sometimes with random litter inside.
What is its purpose?
It's a junction box for stormwater pipes. ELI5: Pipes are straight runs. Junction boxes are structures that allow for changes in direction or elevation of the pipe runs. The pipes get plugged into the holes and everything gets buried.
https://oldcastleinfrastructure.com/product/24-x-24-junction-box-mansfield/