Insulin inhibits or lowers your ability to burn fat.
Insulin is a hormone or signal that notifies your cells that your blood sugar is high. The purpose of insulin is not to make you fat, but it is there to protect you from the toxic effects of having high blood sugar. The effects on body fat and the metabolism of body fat are a response or means to protect your body from these effects.
The toxicity of sugar is called glucotoxicity. Untreated high blood sugar is toxic and causes glucotoxicity. The cells in your pancreas produce insulin as a means to moderate the elevation in sugar after meals. Too much sugar can acidify so signaling your cells to pull sugar out of your blood so your cells are the mechanism to reduce this. This process also helps to regulate your blood sugar levels.
The problem is that insulin also signals your cells to store this excess sugar as fat. This process is called lipogenesis. If you are storing, it would not be beneficial to also break down fat, lipolysis, so insulin also turns off the fat release for energy and thus reduces the rate your body burns fat.
Insulin inhibits lipolysis because you just consumed food that is providing calories or energy. The carbohydrates, fat, and protein are there to be used for energy and body maintenance. For this reason, insulin elevation chronically can induce obesity.
The bottom line: Insulin spikes promote fat storage, reduce fat utilization for energy, and increases obesity. It makes sense to reduce sugar spikes and simple sugar intake when you are trying to lose weight.