Why? Because Windows 7 offers a perfect balance between the modern driver support of Windows 10 and the low resource consumption of Vista. But without the correct drivers, your PCG-31311M will suffer from poor display resolution, no Wi-Fi, broken audio, or malfunctioning function keys (brightness, volume, etc.).
The (often recognized as part of the Vaio VGN-FW or NW series) is a legacy laptop that originally shipped with Windows Vista. However, many users still prefer or need to run Windows 7 on this machine. Sony Vaio Pcg-31311m Drivers Windows 7
Check the Hardware IDs in Device Manager and comment below (or search for "Windows 7 driver for PCI\VEN_..." on driver forums). or malfunctioning function keys (brightness