The headphone amplifier makes sure you get the most out of your headphones
Many PC sound cards and other audio sources are of poor sound quality or do not provide enough power for more hungry headphones, which may be to the liking of a hi-fi enthusiast. With a headphone amplifier, you are sure to get enough power and quality out loud.
Connection options should also be considered. Many amplifiers have options for several different sizes of headphone jacks. For example, a typical connection is a 6.3mm or 3.5mm TRS stereo connector and a more special option is a balanced 4-pin XLR, a 2.5mm TRRS connector or a 4.4mm Pentagon. Versatile connection options for other devices are also typical.
DAC or external sound card and amplifier
Some headphone amplifiers already have a DAC converter itself, ie an external sound card with which you can convert a digital audio signal, for example from a PC via USB, using Spotify as a player, for example, to analog. Often, these devices today already have the quality and features sufficient for most users, so a separate DA converter is not required. However, a separate DAC often greatly increases connectivity.
Preamplifier also included
Often, headphone amplifiers also provide a line-level output for the audio signal, in which case the device acts as a preamplifier and can be connected directly to, for example, Genelec's active speakers or a power amplifier for passive speakers. This is often a good solution for desktop users who listen to music on a PC or Mac.