Why can butter but not all spreads be used in cooking?

The consistency of butter is a natural emulsion, whereas the fat and water content of many manufactured fat spreads is achieved by the addition of emulsifier ingredients to bind the spread together. These are often less stable when heated causing the composition to break down, leaving the solid components more prone to burning once the water fraction evaporates.

In addition, most fat spreads are much lower in fat than butter, making them unsuitable for cooking that requires fat as the medium, such as frying. Regular margarine can be used in the same way as butter during cooking due to its similar fat content.