PCIe primary purpose was to provide a standardised expansion bus/slot Type suitable for everything.
by the time of PCIe then thankfully the various ISA, EISA, VL-bus systems, and MCA (Reiner those) had gone away and left with PCI, PCI-X and AGP being around. AGP being developed as PCI starting to show limits for cards.
so would end up with high end boards where has AGP slot for Graphics card. PCI-X for 10gb NIC/Storage Controllees, PCI for sound cards, 10/100/1000 NIC, etc.
PCIe replacing all 3 slot types allows greater flexibilty in that only one expansion bus to cater for and the PCIe lanes could be divided up how the system board vendor wanted too Based on the number of lanes the controller supported.
As computers got more integrated and the built in on motherboard parts got better then less expansion cards needed for the majority of people And so ended up where the majority of PCIe cards installed by users into desktops are GPU cards but that is more the fact that people don’t have too buy there Network Cards and Sound Card seperately these days for the majority of people. Wi-Fi and BT again pretty much onboard and not expansion slot. 4 port NVME storage cards again for the majority of PC’s are not being installed.
can even find 10gbe built into higher end boards theses days rather then requiring to go out and buy a 10gb NIC card.