I found a flourless chocolate cake recipe that I want to try, it uses semisweet chocolate which I thought would have enough cocoa solids already but it calls for cocoa powder, why is this?
You can't add too much chocolate into a cake, as you'll end up adding too much fat, which will change how the cake sets up. They likely wanted more chocolate flavor than what they felt they could get from chocolate alone, and so opted to add cocoa powder as well.
Cocoa powder also adds starch, which helps with structure. The plant stores starch in tiny, hard granules, but these granules swell and absorb water as they heat up (when your cake hits the oven). Eventually, they swell too much and burst, releasing individual starch molecules into the liquid around them. These long molecules tangle with each other and create a mesh-like framework that prevents free movement and helps turn your cake batter into a solid.
Chocolate also contains these starches, but solid chocolate also contains fat, sugar, and often milk or other additions. Adding these components individually gives you more control over the flavor and texture of your cake.
More about cocoa powder and its role in baking at my blog: http://www.fchem101.com/2015/01/cocoa-powder-and-cookies/