Feeling dumb re: sleep training

SweetLittleThing

I feel so out of the loop.  What is it? Why do it? Do we need to?

LO falls asleep after her last bottle about half of the time and will wake up todrowsy state as we put her sleeping bag on and put her in the crib, but then sleeps for a minimum of 6 hours and normally it's 7.5. 

The other half of the time I put her in the crib fully awake and within 15-20 minutes she's asleep.  She doesn't cry, she just plays with her hands or talks to the Snoopy mobile until she's out.  

we have never swaddled, we don't use white noise or any other soothers in the room.  Occasionally she will get a paci, but never to fall asleep, only if she wakes up more often than normal and I know it's not time to eat.  

What is sleep training for?  Does it help them sleep longer? Or sounder?  I need help