Advantages with parking a domain:

For the most part the broker want you to park the domain with them but it is not mandatory in most cases. There are soem advantages with parking a domain. For most people, the first step in buying a domain is simply to type the domain they want into their browser. If the domain is parked, they are shown a “For Sale” page where the visitor can quickly make an offer through a credible third-party.
Most broker's will put in extra effort to expose your domain on their website if it is parked with them.

Disadvantages with parking a domain:

1. The website content you made will disappear.
2. The broker will rake in most of the pfoits from using it for advertising. AT best you get some percent of the advertising incomes.  In other words the broker is enjoying the advantages of "your" traffic.
3. Parking a domain with one broker means you cannot promote it at other domain trader sites.