This seems to be a temporary folder used when installing the Engine. I had issues trying to install to a volume with limited space — the install doesn’t complete, when the space runs out, and hangs. Opening, listing or removing the .egstore takes a long time (try rm -vrf for the folder), since it is filled with small snippets of data, which I guess are used during install process. While the downloaded files are about 18GB (for version 5.5), the installed files PLUS the temporary files take way more space than what is mentioned in the Epic Games Launcher. Installing seems to take many hours, if you are lucky enough to get it to work…So far I am stuck with 5.4, trying to think where to release 200GB in my main drive to allocate hopefully enough space for the install.