Headphone users make up a large segment of music listeners. As headphones have steadily improved in quality - especially in the past 5 years - they will reveal the short-comings of basic headphone outputs in preamplifiers, integrated amplifiers, receivers, and portable music players. In addition to providing low distorion and low noise floor power to drive the more demanding headphones, headphone amplifiers are available with high quality DACs and with volume control and ability to drive power amplifiers without the need of a preamplifier. Sophisticated high-end designs feature balanced outputs. Tube designs offer a slighly different listening experience.
There are some excellent portable headphone amplifiers with the means to sustain output for long haul plane flights.
You can get much better sound with a headphone amplfier with a high performance DAC than directly plugging into your laptop. Bring your current cans along for a demo.