>>719199843
It was originally supposed to be an organ that produced the T-Virus. The whole idea was that it could control the zombies created by the virus. That's also what those Bible verses in the lab talking about how your offspring will outnumber grains of sand or whatever are referring to. I think those ideas got reused as that one monster spewing the virus in RE6 and Eveline making/controlling the molded in 7.
You can see in the concept art it's not really even a heart. Just a big misshapen organ with "arteries" ending in little mouths and that even carries over to the game model. All that stuff got dropped pretty late in development and it's just a big naked gray guy.