If you cook a lot you should have most of these spices in your cupboard. Mixing them yourself is much cheaper and you can be more creative with the proportions and amounts.
I usually mix of couple of these blends ahead of time so there is less prep time when you make your meal. The taco mix is great with ground hamburger or pork. The Italian seasoning is nice for an Alfredo sauce or add it to tomato sauce for a simple spaghetti sauce. The Cajun mix is very versatile and can be added to fried potatoes, farmers omelets, egg bakes or try it in Alfredo sauce. The Greek mix is a nice rub for meat to be roasted or grilled.
The infographic is from twistedsifter.