For me, working out with tailored music dates back to when I trained in my garage gym a little over 10 years ago. I'd put on my favourite Rammstien CD and get psyched up. I kept the angry vibe going by working out on the speedball or boxing bag between sets. Generally I've always lifted weights to the soundtrack of loud music, the angrier the better; however, a few months back I stopped listening to my usual play list (headphones broke) and used the music of the gym, which is generally whatever is playing on V or Max on PayTV. After a while I noticed my gains weren't coming and I wasn't lifting as much as I used to. Then I remembered the last time this happened, and the cure was to go back to blasting my ears with my usual playlist.
Working out to your own playlist allows you to customise the music to what really motivates you. Some people find energising dance music helps give them a boost, and I'm sure others might benefit from even classical music. I guess it depends on personality.
Wearing earphones also has the added benefit of allowing you to ignore anyone who might talk to you. I come to a gym to workout, not to socialise, and getting caught up in 5 minutes of conversation not only wastes what little time I have available, but cools me down and takes away my focus. With earphones in, I ignore whomever might be trying to strike up a conversation with me. Easy to do if you can't hear them!
I've used the same ordered playlist for so long now that I time my workouts by it. It seems like my body knows what I'm about to do based on the song that's playing, or about to play, and it responds accordingly.
Something else I was considering last night was stress. Loud music is stressful, and playing loud music may possibly give a small boost of adrenaline that you not get if the music volume was less.
I personally experience about a 10% increase in the weight I can lift each rep (with the same rep and set volume) when using my personalised playlist.
Here's my current playlist:
||Freak on a Leash, Got The Life, A.D.I.D.A.S., All in the Family
||Fuel, Enter Sandman, Wherever I may Roam
||Bestrafe Mich, Weisses Fliesch, Wilder Wein, Du Hast, Laichzeit
||Back to the Primitive, Mulumbo, Jumpdafuckup, Umbabarauma
||Yellow Leadbetter, Black, Porch, Why Go
|Guns 'n' Roses
||Sweet Child O' Mine, Out Ta Get Me, You Could Be Mine
||Best of You