The whole point of someone buying anything established is the idea that they are buying something that is already profitable. The same thought process applies when it comes to buying established Adsense sites for sale. A buyer knows their money is made by the number of visitor clicks they receive on certain advertisements within their website.
Of course once they decide to purchase a site they are free to add, remove or change any content that they desire. Some buyers will not make any changes to an existing site while others may try to enhance the site to make it even more profitable.
It is very common for buyers to purchase their established Adsense sites for sale from a web-development company. Making a purchase from a company of this nature provides certain benefits to the buyer such as the ability to host their site with that company, continued support from the company and possibly some free hosting privileges for a pre-determined period of time. If a buyer is new to this type of business ownership this may the perfect way to get some experience while still having support available whenever they need it.
These types of purchases are attractive to buyers because it gives them the chance to start out with an already profitable business. That being said, they have the ability to change the content of their website at any time, which means they are in total control of what type of information is being portrayed to their potential customers. For some people the ability to have this type of creative control is priceless. What makes it even better is that they did not have to spend a ton of time developing the product from scratch because the majority of the work was already done for them. Every buyer has different preferences for how they do business but they all like to make a profit.