[speeder]

I am assuming you don't want to know why the whole mission is important (that should be obvious), and that instead you want to know why taking a photo of Philae is important.

It is important because Philae did made measurements and sent the data to us, but we didn't knew what the measurements measured, now we know.

It would be like throwing a ball that can tell about how much a place is wet in a random direction, and conclude that some place is 90% wet... Then, what place it was? A lake? A swamp? A beach? And then you find a picture of it in a bog, and conclude the bog was 90% wet.