Your built-in flash:

+ it is always there
+ no cables necessary
+ If the camera battery is charged, the flash is charged
+ Often has through the lens metering for exposure
+ Can be used to trigger external flash units
+ can provide nice fill light
- Has potential for red-eye
- if it is only light source- produces flat- lifeless light
- if it is only light source- can cast un-natural shadows
- without filters has daylight balanced light only