One of the most useful accessories for the Raspberry Pi mini PC, laptop, and desktop PCs is a USB 2.0 Hub like Sabrent 4-Port USB 2.0 Hub with Individual Power Switches and LEDs (HB-UMLS) because it adds 4 or more empty USB 2.0 ports to the computer. A USB Hub is very useful if you plan on using the Raspberry Pi as a mini desktop computer because 2 USB plugs are usually not enough if you plan on also plugging in a USB flash drive, USB external hard drive, Wi-Fi USB adapter, Bluetooth adapter, and other USB accessories which work with the Raspberry Pi. The 2 USB ports are not enough since they are usually use for the USB keyboard and mouse.
The Raspberry Pi’s 1 USB port can power a 4 port USB 2.0 hub with no power adapter which also has a keyboard, mouse, a wifi adapter, and 1 flash drive plugged into the hub in my experience. But, if you plan on also plugging in a USB fan, USB hard drive without a separate power adapter, and other high powered USB accessories, I recommend a powered USB hub.
USB hubs are easy to use. I only need to plug in the USB cable from my hub into my computers USB hub, and I now have 4 or more USB ports instead of 1.
Some USB hubs let you turn off the connection, and power to each individual USB port on the hub, so you save a small amount of electricity, and free up bandwidth, so copying, and writing files to and from your USB storage devices is faster.
USB 2.0 hubs are also backwards compatible with older USB 1.0 devices, and USB 3.0 is backwards compatible with USB 1.0-2.0 ports. Most USB hubs in my experience work with the Raspberry Pi, Windows, Mac, and Linux based computers.
USB hubs are usually pretty cheap when I buy them online, or at computer stores because they cost $20 or less.