What information do bipolar cells encode?
Recent nature neuroscience review on bipolar cells says that bipolar cells are classified into 13 (12 cone bipolar + 1 rod bipolar) different types. 

Depending on the polarity of their light response, bipolar cells can be grouped into ON and OFF cells. 

Answering your question, what are on bipolar cells used for ? ON bipolar cells predominantly depolarize at light-onset and stratify in the Inner Plexiform layer (ON). 

Considering rod - rod bipolar path way: Eyes are sensitive to very low light level. Imagine you are seeing a star during night. As the number of photon reaching retinal layer will be very small. It reaches order of few rod photo receptors (receptive field) &amp; then converge to rod bipolar cell for efficient signal processing. It makes vision at dim light really interesting and promising. 


In case with central surround organization, It is one of the classical stimulus study which has been explored further in detail with various complex stimuli.
I recommend you to go through The Neuronal Organization of the Retina for more details.

I am absolute beginner in visual neuroscience field. Please correct me if am wrong.

-Sathish