What is the purpose of having more than 1 PCIe x16 slots?

This is the motherboard for precision t1700 There are 2 slots, PCIe x16 is #16, PCIe x4 is #18 User manual for t1700

The PCI Express x1 to x16 adapter accommodates low profile x2, x4, x8, x12 or x16 PCI Express cards, eliminating the cost and hassle of upgrading a PCIe card or

pcie slot size pciex16 #GPUinstall #howtoinstallgpu #GPU #graphicscard installing any pc component is

pciex16 slot configuration a video card using a PCIe x 16 slot is faster than the video port built in on the motherboard true or false?