Useful if connecting higher end headphones, not useful if you're only going to connect simple low power in ears.

The moment you connect any sort of bigger headphones that do not have their own amplifier to "standard" 3.5mm on most portable devices (even laptops), you find yourself benefitting from such pocket amplifier.

The power available on those outputs tends to be quite low, so attaching higher impedance headphones means the resulting sound is quieter, which results in user cranking volume up to compensate, which makes the usable range of volume smaller for adjustments, but also that often quality noticeably falls down because most amps have more distortion at upper ends of power.

Depending on devices, sometimes you even get noticeable difference in power drain.