Why to use libraries:
- it's probably been tested/used more than your code
- someone else maintains it for you
- probably designed better than you would do it
- this includes stuff like speed, security etc
- saves time & effort (obviously)
- in a scripting environment, native code runs faster than your own version
- sometimes you don't exactly have a choice (eg integration with another application)

For learning, yeah implement it yourself if you want to, or to boost the nerd cred, but in real world situations, it's generally better to use stuff that's already there (if you can).  Main reason you wouldn't would possibly be:
- licensing issues
- you want something that the library can't offer (and you can't simply modify it)