IMO & experience, but with no hard data to back them up, I'd say this really isn't something to be concerned with before you have a true need for that level of leadership and for any smaller company (I can think of exceptions of course). Lots of startups give people the VP title or Director title, and there are only like 5 people in the entire company. I understand the reasons it is done, but at that level they don't require special employment agreements, they could have them but they aren't required. Overall these types of employment agreements are usually done for publicly traded or larger organizations where they need the extra details in writing and those positions are really a VP of something not of 10 people. Personally, I've been in companies where there was 1,000 people and over a billion dollars in revenue and none of the execs had separate employment agreements (which to me was odd). I've also been at a small startup where a specific engineer (nothing special) was given a special employment agreement.
As far as what is in them. There is a lot of boiler plate, including moral turpitude clauses, specific pay agreements, stock options, and any special disclosure or other arrangements. I've had these a few times, and in general they are nothing that special but they do lay things out clearly for both parties, and they are nice to have in a lot of ways because it makes things very clear and relatively enforceable compared to a standard employment relationship. You will find terms in these that a lot of times would not be allowed under traditional employment laws in the U.S., but because the person is technically a contract employee the rules are slightly different (and can vary state to state). In my case, all of them had morality clauses, public disclosure, publicity, association restrictions and very detailed pay, tax & bonus details.