You Should Add More Content To Your Blog When:

1 - Your homepage has lots of backlinks, but few words on it. Then consider to add some “words” on your blog not just a backlink.

2 - You’ve got 50,000 pages, but only 100 have more than 50 words on them.

3 - You’ve got the same content on your pages as 50 other sites have. Google will try to choose the 1 page it thinks is the “real” one, will it be yours? or will yours go supplemental?.

4 - You’ve got hundreds or thousands of product pages that only have a small description. It’s hard enough to keep these from going supplemental, let alone rank for anything (with little content).

5 - There’s nothing on your site that’s “link worthy” from your peers, or from anyone else. Having quality researched content can garner you natural backlinks (gets you backlinks w/o asking for them).

6 - You’ve been targeting 3-4 “big phrases”, but have been ingoring all the other phrases. Having content pages covering lots of 3 and 4 word keyword phrases helps pick up related long tail phrases. Also, focusing only on a few “major phrases” isn’t good “insurance”, and is getting harder.

7 - Google tries to rank “resource pages” high, and “resource pages” tend to have lots of original content. (via JimBoykin)

8 - There only a videos and pictures in all your post. Try to make some valuable post.

9 - Your blog only had “link love post” where all the post pointed to other URL. Some kind of speed linking or link round-up. Then you should create some post that not pointed to other URL.

That why content are the “king” for your blog, any other reason think for yourself.