As has been mentioned before, the actions of insulin are multiple and it is not a steroid. It has a number of key activities in relation to the body's working.
1. It allows the body to use glucose that is ingested.
2. It allows muscles to use amino acids for restoration.
3. It shuts down the body's use of fat as a fuel source and tells the body to prefer glucose.
Due to these actions, insulin is sometimes described as a fat gaining hormone, when in reality it is simply stopping the body from burning fat as energy, causing more food sources to be converted to body fat.
If you lower the amount of insulin in the body, it is more likely to burn fat and thus you lose weight more effectively.