Why would someone need a headphone amplifier?

You need one primarily for high-impedance or low-sensitivity headphones that your source device (phone, laptop) can't drive adequately. It prevents distortion, boosts volume, and improves overall sound quality and dynamics.

What is the difference between a DAC and a headphone amplifier?

A Digital-to-Analog Converter (DAC) converts digital audio signals into analog sound. A headphone amplifier then takes that analog signal and boosts it to power headphones. Some devices integrate both functions.

Can a headphone amplifier improve the sound quality of any headphones?

While it significantly benefits high-impedance or low-sensitivity headphones, a good amplifier can subtly improve clarity and dynamics even with easier-to-drive headphones by providing a cleaner, more powerful signal.

How do portable headphone amplifiers differ from desktop models?

Portable amplifiers are compact and battery-powered, designed for on-the-go use, often with less power output. Desktop models are larger, typically AC-powered, and offer more power, features, and connectivity for a dedicated setup.