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.
‘Sec. 2631b. Supplies: preference to United States aircraft
‘(a) Preference- Only aircraft owned by the United States, or aircraft operated by or under the supervision of United States air
carriers holding a certificate under section 41102 of title 49 and
registered in the Civil Reserve Air Fleet, may be used for the
transportation by air of supplies on behalf of any component of the
Department of Defense. However, if the President finds that the rates
charged for the use of those aircraft is excessive or otherwise
unreasonable, contracts for transportation may be made as otherwise
provided by law. Charges made for the transportation of those supplies
by those aircraft may not be higher than the charges made for
transporting like goods for private persons.
Executive orders are abused by lame duck presidents and those without support in Congress. There is nothing preventing the next president from reversing every executive order the previous occupant ever wrote while the outgoing occupant prays that the judges he appointed won't flip. I view it more as symbolic political gamesmanship than a serious governing tool.