You take one shot of it for your long acting
insulin, and you still need to take a fast acting
insulin to cover your corrections and meals. It is provides a fairly constant basal insulin all day long. 70/30 mix has both, fast acting insulin to cover a meal, and long acting to provide that basal between meals.
Here is a listing of different types of insulins and their details:
http://www.childrenwithdiabetes.com/d_0n_110.htm
To me, the basic difference is variability and flexibility.