Why do RAM sticks need to be separated in the DIMM slots?

Why do RAM sticks need to be separated in the DIMM slots? dimm 2 slot 2 x DIMM, Max 16GB, DDR3 Non-ECC, Un-buffered Memory; New UEFI BIOS & USB Features: 3x slot, Realtek Ethernet 23,600৳ 24,800 dimm2 slot DIMM sockets Up to 192 GB with 16 GB DDR3 RDIMMs and 12 populated DIMM slots

dimm2 slot Support 2-DIMM DDR4-1866213324002666 up to 32G maximum capacity; Support 12 x PCIe slots; Support Connector; Support HDMI Connector PDF

dimm slot However, according to the hardware specifications for my laptop, there are only 2 x SODIMM slots available for use I have an additional 1TB SSD MSI 7883-001R Computer Dimm Lga 2011-3 Motherboard X99A Godlike Gaming Carbon, e atx 2 Slots, Ethernet, USB Type-C, Thunderbolt 4 Support and RGB