Because no insulin injection is immediately 100% absorbed by the body, the yet to be used insulin stays under the skin, the system drawing on this "reserve" as needed. Any such insulin effects that last after the insulin's expected action is over are also known as carryover in some literature.
The larger your insulin dose, the larger the insulin depot in your subcutaneous fat tissue.  Having a large vs smaller insulin depot means the effect of the insulin can be less consistent--more chance of it varying from day to day.  Dr. Richard K. Bernstein,  father of the modern Glucometer,  also makes a valid case for this,  as does Dr. Hanas.  This same insulin depot is the reason why it can take anywhere from 2-5 days to see any effect of insulin dosage increases. 
Intervet also addresses the issue when stating that the dose per injection on a twice-daily regimen is less (then the large once-daily injection), which results in less hypoglycemia and better glucose control. 
This handy "extra tank" is also there to give you a hand if you miss or are late with an injection.
On the other hand, when you do miss an injection, your "spare tank" is very low by next injection time, and so the next injection may have less effect than expected!
This free downloadable chapter  of Dr. Hanas' book contains an easy to understand explanation of an insulin depot on pages 10 & 11.
- ↑ King, Scott (28 November 2007). Why Smaller Shots of Insulin Get Absorbed Faster, Peak Sooner, and Are Out of Your System Quicker. Diabetes Health.
- ↑ Higgins, Thomas. Insulin. Boulder Medical Center.
- ↑ 3.0 3.1 Bernstein, Richard. The Laws of Small Numbers-Part 2. Diabetes In Control.
- ↑ Mendosa, David. David Mendosa's Page on Dr. Richard K. Bernstein's Accomplishments. Mendosa, David-mendosa.com.
- ↑ Hanas, Ragnar (1999). Insulin-Dependent Diabetes (Pages 10 & 11). Children With Diabetes.
- ↑ Herrtage, Michael (2009). Canine Diabetes Mellitus-Insulin Therapy-. WSAVA.
- ↑ Richards, Mike. Insulin use in Dogs-Regulating Insulin. Richards, Mike-Vet Info 4 Dogs.
- ↑ Caninsulin-Starting Insulin Therapy in the Healthy Diabetic Patient (Page 4). Intervet.
- ↑ Hanas, Ragnar (1999). Insulin Dependent Diabetes In Children, Adolescents & Adults. Children With Diabetes.