This was a really interesting read (thanks samwize for the link). The shorty summary is that depression may be tied to brain cells not growing/repairing properly, and thus have a traceable and sometimes fixable physical cause. It discusses some of the research that those conclusions came from, and why it may be a more realistic understanding than traditional "how you feel" approaches.
If they can uncover the underlying physical contributions to depression, treatment can be far better targeted and possibly far more effective. Just as importantly - this understanding reduces stigma; depression has emotional symptoms but its cause may be cellular just like any physical illness.