USB host cards, also known as USB host controller cards or USB expansion cards, are addon components that provide additional USB ports and enable a computer or other devices to connect to USB peripherals and devices. These cards are designed to expand the number of available USB connections when the existing USB ports on a computer are insufficient or if additional USB ports are required.

USB host cards come in different form factors, such as PCIe (Peripheral Component Interconnect Express) cards for internal installation or external USB hub-like devices that connect to a computer via USB or other interfaces.

USB host cards are especially useful for desktop computers, laptops, and servers that may have a limited number of built-in USB ports. They are also handy for users who frequently work with multiple USB peripherals or need to connect and disconnect devices regularly.

When considering a USB host card, it's essential to verify compatibility with the computer's operating system and ensure that the card provides the required USB standard and number of ports to meet your specific needs.