Ex. Is when say movie critics who watch and and review movies before they come out. The critics have to sign a contract not to give out any spoilers about the movie.
It is deemed illegal if there is someone under the age of 18 it technically isn't a legal contract because they arent of age. 
One example of a profession that uses contracts on a daily basis is a Banker because they have new clients coming in daily and they have to make up contracts to make sure the clients money will be safe. Another profession is a Real Estate agent when they have to sign contracts with the people who are buying houses.

The most important elements of a valid contract are offer, mutual consent, competency of parties, lawful object, possibility of performance, and other legal formalities.  Offer is the process of formalizing the object of a contract. Mutual consent is pretty obvious, both parties must be in lawful agreement without any coercion.  Competency is also relatively self explanatory; both parties cannot be minors, must be of sound mind, and cannot be legally unable to enter a contract.  The object of the contract must be legal, for instance, I can’t enter into a contract with the object being someone’s murder, because murder is illegal.  The contract must be possible to be completed, for example, I can’t enter into a contract that requires me to fly without mechanical assistance, because flying without mechanical assistance is impossible.  Other legal formalities include which contracts can be verbal and which contracts must be written, for example, to buy a house the contract must be written.

Banks, Insurance companies, and any business that has warranty policies use contracts on a daily basis.

https://gradestack.com/Mercantile-Law-for-the-CA/Nature-and-Types-of/Essential-Elements-of-a/22678-4473-55812-study-wtw