mberlin74, the simple answer to your question is that with the V6, you don't "need" an amp. They are easy to drive with all sorts of equipment and sound good, that's why they are ubiquitous in the recording industry. There are other phones that do "require" an amp to sound good. The V6 doesn't.
Now that we got "needs" out of the way, let's talk "desire". When I listen to music, I like it to sound as good as it can sound. That's where the amp comes in. As joelongwood said, the amp delivers better quality power to the phones and controls the drivers much better. This results in tighter bass, better mids, and more extended highs.
There are many different amps out there that are designed with different needs in mind. joelongwood, suggested the CHA47. This amp was designed specifically for headphones with low impedence. Low impedence headphones need a higher amount of current to drive them appropriately. The CHA47 is an inexpensive, portable amp that should work well with the V6. High impedence headphones, OTOH, need a higher amount of voltage to drive them well, so a different amp will be used for them. The CHA47 will still work, just not as well as some other amps with high impedence headphones. Another amp that is mentioned here very often is the META42. This is a higher priced amp and is designed to deliver high voltage and high current and will work well with most HPs. The META42 design is versitile and it can be customized for specific HPs.
Headphone amps can be used with almost any source, but using a "line out" would get you the best quality.
Good luck with the V6 and welcome to the world of headphone listening. Be sure to post your opinions of the V6 when you get them.