I am thinking of buying a usb wifi adapter but i'm not sure if it'll be useful when im using wsl
Windows Subsystem for Linux (sometimes called Bash for Windows) inherits networking from Windows. Just type "ip addr show" and you will see the same networking information as Windows. Linux programs in WSL have the same access to your local network and the internet that Windows programs have.
You can certainly add a USB WiFi adapter if you like. It should work for both Windows and WSL. However, WSL will not see it as a USB device or be able to control it directly. It will only be available for networking. If you really want to use the USB WiFi adapter with Linux drivers, you will need a full Linux virtual machine such as Virtual Box.