It may be fairly obvious that family is extremely important. They are the people who witness your constant changes over time, even being part of the cause behind those changes sometimes. Because you share share with them, family members will always hold some part of your heart in their hands. It's inevitable. However, there are certain things that family members just can't give you the same way friends can.
For one, a friend's love is something they choose to give out. When it comes to family, they are supposed to be the people that love you unconditionally and without restraint. Friends, on the other hand, have no obligation to care in that way. Instead, they freely choose to love you. That speaks volumes compared to familial affection. A friend is someone who decides that, no matter your flaws and oddities, you are worth loving. That's a type of affection family just can't quite give to full capacity, because even if they do believe you are worth loving, there is always going to be that underlying factor of family being expected to love each other.
Then there's also the matter of friends being closer to your age. Yes, siblings can be around the same age, and parents have (at some point) been your age, but friends are the kind of people that are dealing with relatively the same issues at the same time as you. They are that reminding force that lets you know you aren't alone in your struggles.
Along those lines, because friends understand what you're dealing with, they are better equipped for helping you through these problems. Family members might offer good advice and want to help, but (because of age differences or lack of understanding) they may not know exactly what is best.
And then there's the fact that friends are there to listen whenever you need to vent about your family. Let's be honest, family members argue and get on each others nerves. As human beings in a relationship (even one built on familial love), you are bound to butt heads at some point or another. And it's not like you can complain about your mother to another family member; that just only leads to more arguments and people choosing sides. Friends, however, will be there to listen without causing more confrontation. That way you get let out steam without causing more trouble. Even better, they are a little more bias in your favor. So if a friend tells you you might be in the wrong in the argument, then you know for sure that you should rethink what you said.
Good friends are going to call you out on crap you do. They know your good sides, so when they see you acting wrongfully, they are going to push you back on the right track.
That said, sometimes family trumps friends. But friends are everywhere and anywhere you decide to look for them. Friends are the family you choose to have.