Executive orders are not permanent, they exist at the will of the president and can be changed at any time. Partisan orders would be canceled when an opposing party president takes office, making them much less useful for permanent policy changes than signed laws from congress. The supreme court has also ruled that is has jurisdiction over any order issued and has struck some down, notably this case which set the precedent that attempting to create law with an order is unconstitutional. They exist because the president and the executive branch is responsible for enforcing the laws of the land and are given some leeway in how to enforce said laws, they have the full force of law because they are issued by the people enforcing the law. Executive orders in modern times are supposed to justified by existing law and essentially be a clarification or explanation on how a particular law or set of laws will be enforced, though there are many examples of orders going beyond this limit.