If you are looking to purchase a specific unit or find technical documentation:

Standard computer sound cards or integrated motherboard audio often lack the power to drive high-impedance headphones or large passive speakers effectively. An audio amplifier bridges this gap by taking the low-power "line-level" signal from your PC and boosting it to a level suitable for driving speakers.

Many modern amplifiers also act as a filter to restrict bandwidth and eliminate interfering noise from the computer's internal components.

The most common way to integrate an amplifier into your setup is via the jack (usually green) on your sound card.