what are m.2 slots used for - Gaming laptops with 2 SSDs? slot mega 888

Most laptops nowadays come with an M.2 slot for an SSD. If you want to clone M.2 SSD to another one, you are required to have two M.2 SSD slots.

PCIe x4: These have four PCIe lanes, and they also can fit into a x16 slot. Often used for single M.2 NVMe SSD expansion cards, they are also used for SATA 3 expansion cards and high-speed network adapters.

The M.2 slot is usually filled by an M.2 SSD but can also be used for other purposes.