It allows you to change the data and group elements into new ones (existing in the data but not in DOM), updated (existing both in the data and DOM) and remove

It allows you to change the data and group elements into new ones (existing in the data but not in DOM), updated (existing both in the data and DOM) and removed ones (only in DOM). Then it lets you to do some cool stuff on them, like transitions: http://bl.ocks.org/mbostock/3808218 http://bl.ocks.org/mbostock/3808218 http://bl.ocks.org/mbostock/3808221 http://bl.ocks.org/mbostock/3808221 http://bl.ocks.org/mbostock/3808234 http://bl.ocks.org/mbostock/3808234