There's a weld joint on the inside, so I guess that patch is to reinforce it.
Sample item: 00 01 02 03 2000-2003 SUZUKI GSXR 600 750 MAIN FRAME CHASSIS W/ PAPERS EZ REG | eBay
Sample item: 00 01 02 03 2000-2003 SUZUKI GSXR 600 750 MAIN FRAME CHASSIS W/ PAPERS EZ REG | eBay