The thermal pad conducts heat from M.2 SSD to M.2 heatsink, to keep M.2 PCIe SSD under good performance. In addition, the size of the M.2 SSD is compact, most are 22x80mm. Manufacturer glotrends. Even if the surface of M.2 SSD is not flat, M.2 silicone thermal pad (20x76mm area) can cover the entire surface and cover closely all the heating electronic components.
eBay