The idea is that they provide more support for large graphics cards, to keep the plastic slot from cracking or outright breaking under the weight of heavy cards.

Lastly, a note about metal-wrapped PCIe slots: These are increasingly common in high-end and even mainstream boards. Even if a smaller board has all the slots and ports you need now, it’s good to have some left over for future upgrades. If you are planning on installing lots of expansion cards, you’ll want to opt for an ATX motherboard (if not an E-ATX model), as smaller form factors have less room for slots (as well as other features that take up lots of PCB space). So while there are six slots on the board here, if you install two graphics cards, you’ll have just two accessible slots to add other cards. In the image above, if you install a gaming graphics card (the vast majority of which are at least two slots tall) in either of the two x16 slots, the shorter x1 slots below them will be blocked. Also know that slot arrangement/spacing is key if installing more than one or two expansion cards. Note that some PCIe slots may be wired for fewer lanes than the slot length suggests, or have some lanes disabled depending on what other slots or fast drives are installed.

Desktop black ram vs blue slot