Most of us use one or two M.2 slots on our motherboard to install NVMe SSDs, and call it a day. After all, storage is what M.2 slots are known for. However, that's not all they're compatible with. You can realistically use a vacant M.2 slot on your motherboard to install one of several types of expansion cards, some of which I've already covered in the article about non-SSD devices compatible with an M.2 slot. In this list, I want to cover some other expansion cards you can install instead of wasting your spare M.2 slots.
5 Wi-Fi adapters
Bringing wireless to your old PC
One of the features that's most commonly absent from some older (or cheaper) motherboards is onboard Wi-Fi. My existing motherboard has it, but the one I bought back in 2017 didn't. If you wish to bring wireless connectivity to your PC without changing your motherboard, or pinning your hopes on USB dongles, an M.2 Wi-Fi adapter is perfect.
You can buy one after checking the type of M.2 slot present on your motherboard (M key or A+E key). Most Wi-Fi cards have Bluetooth as well, so you can get two features for the price of one. If you wish to buy a Wi-Fi-only card, you still have that option.
4 Ethernet cards
Multi-gig wired speeds
Ethernet would already be present on your motherboard, but all Ethernet ports are not created equal. Maybe you have a Gigabit port, and need additional bandwidth to accommodate your server or home lab needs. A 2.5GbE or 10GbE card would instantly speed up your network capabilities, allowing you to shuttle large files and media across your devices at breakneck speeds.
These Ethernet cards can be quite useful for power users and home lab enthusiasts since buying a new motherboard for multi-gig Ethernet isn't always feasible. If a cheap M.2 Ethernet adapter can provide you with the same capability, then there's no need to spend more than you need to.
3 Capture cards
Don't sacrifice those precious PCIe slots
Some manufacturers have started producing M.2 capture cards for SFF PCs and embedded machines, utilizing the PCIe 2.0 x4 configuration. These are relatively rare and expensive, but if you don't have spare PCIe slots, or don't want to occupy them with a capture card, these M.2 cards can come in handy.
Many users want to stream gameplay without sacrificing performance, and dedicated capture cards fill that gap nicely. Since they use PCIe speeds without blocking the PCIe slots, they can be better than USB capture cards.
2 SATA and NVMe cards
When you can't enough storage
While SATA and NVMe expansion cards still count as extra storage, they serve an important purpose on some builds. Maybe you've run out of all your SATA and NVMe ports, and want to connect additional hard drives or a few NVMe SSDs. M.2 to SATA or M.2 to M.2 adapters can expand your PC's storage capabilities by providing additional SATA or M.2 slots on a single card.
Users who don't want to buy a dedicated NAS might find such adapters useful for installing more storage drives than their PCs are otherwise capable of.
1 USB adapters
How many USB ports do you want?
You might have seen PCIe to USB adapters on PCs running out of USB ports. Well, if you want extra USB ports without gobbling up your PCIe slots, you can opt for an M.2 to USB card instead. Many of these expansion cards convert a spare M.2 slot into a USB-C or even USB-E port, so you can use it for specific use cases.
You might not get the 6 or 7 USB ports on a single card, like you do with PCIe cards, but M.2 to USB cards can still prove useful. Maybe your case has a USB-C header, but your motherboard doesn't. You could use an M.2 card with a USB-E port to bypass this deficiency.
For those who want to populate every motherboard slot
Some users have an irrational need to fill up every motherboard slot they can, or maybe they just want to add additional capabilities to their PC. Whatever the reason, M.2 adapters can come in handy if you want to add Wi-Fi, multi-gig Ethernet, or additional ports to your motherboard. Not all of them come cheap, but depending on your setup, they can save you the cost of a new motherboard.