I asked this same question when I went to the feed store the first time. The guy's answer? Cost. There may be something special in all these private label seeds, but you may or may not see it. If you're just starting out, I would recommend you get your feet wet with a simple trial-- wheat and clover as a for-instance.
But there's a catch. Clover and wheat are okay, but overall be careful of blends. More often than not, when you mix seeds together, one seed will sprout and crowd out the others. I once thought I'd be smart and put sunflowers in with the rest of what I was planting-- bad move. Some plants like sunflowers included exude toxins make inhibit the growth of other plants. If you're going to mix things up, it's often a good idea to do part of the plot in one thing and part in the other, rather than mix it up all together. That way, one variety won't crowd out the others.
My advice is:
1) Invite your state wildlife biologist over for a survey. Mine came for free, and his help was invaluable.
2) Go to the feed store and ask for advice. Remember, he's going to be directly rewarded by giving you the best advice, because he knows you'll come back with repeat business if he steers you the right way.
3) Take some time and study up on things yourself
4) Ramp up slow and easy. I tried a whole bunch of things and went to a lot of expense before finding out a simple plot of ladino clover and wheat did better than anything else.
When I first got into this, I was thinking I'd go with one of the fancy name brands. I went to the feed store, figuring I could do better mail-order, but just listening to what he had to say. The guy at the feed store had the fancy name brand I was thinking of at a good price, but he said he could match it pretty well at a cheaper price with better stuff that he had on hand. He also cut out a few components that probably would not come up due to my soil pH.