Hopefully this is not a repost...please delete if so
Well, it's fairly simple really. You get a friend who has some battle pets and you can pet-duel eachother and boost for achievements. You can also "trade" your pets to one another and it will count towards your unique pets achievement.
Some things to note:
1. You do not get experience from this.
2. It's only real use is for farming the achievements. For example, you can use this to get the win 5 PvP pet battles in a row.
Achievements that I have found can be used by doing this:
1. Win 5 PvP pet battles in a row w/o having one die.
2. Win 250 Pet Battles (or whatever one you're on).
3. The amount of unique pets you have collected. To do this, you simply uncage one of the pets that your friend needs. Trade it to him, and have him cage the pet. Once you do this, it will count to your total number of unique pets collected. Simply trade it back, and wa-la. (Note that some pets cannot be caged)
4. It also counts towards the number of PvP Pet Battles you have won.
There is some sort of bug in the achievement system where when it keeps track of the number of times you win, or the number of unique pets you have gained, it won't say that you have done it. Ignore this, it will still give you credit for the achievement even if it doesn't say so at the start. (For example....say you win 1 match without having a pet die...it might say that you're still at 0/5 in the progress, but you are really at 1/5. Keep going until you beat him 5 times and you'll get the achievement)