What is PCIe (Peripheral Component Interconnect Express)?
Every high-performance computer motherboard has a number of PCIe slots you can use to add GPUs, RAID cards, WiFi cards, or SSD (solid-state drive) add-on cards.
memory - PC doesn't boot after RAM replacement - Super User
With two RAM slots populated, you have the option to add more RAM later if needed. You can simply add another two sticks of 16GB to reach 64GB total.
What Is a PCI Device? | Inspired eLearning Blog
As you can see, PCIe slots offer several useful ways to step up your PC and add new functionality. If you have empty PCIe slots on your motherboard, consider filling them so you get the most out of your system!