In case of getting data directly from a data table, every time you are required to retrieve the data, you should make a DB call and retrieve data from data table. If it is required to retreive data from data table for every work object, there will be those number of calls to DB which will be a preformance impact. To avoid that, Pega introduced a data page, where using load activity, you can retrieve data from data table once and store it in data page.
You can refer this data page any where in application where you wanted data which will not have a DB call and loads data. This data page canbe given scope of Node so that the same data will be used by all work objects and users across the node.
In addition to caching data (incidentally, Data Pages also have a configurable Refresh Strategy that you can use to ensure that the Data in the Data Page does not go stale); Data Pages also act as an important layer of re-direction or abstraction between how we get data (the Source of the Data Page) and how we use that data.
By separating how we get the data from how we use it through the Data Page we can minimize the ripple effects of code changes. If you don't use a Data Page and you should need to change the data source (like switching from a Report to an Activity) you will have to go to all the places that used the Report and reconfigure the artifacts that referenced the report to now use the Activity instead. You can avoid this maintenance headache by using a Data Page. If you need to modify or change the Data Source, you only have to do it in one place, the Data Page, as opposed to making the change in many places. This simplifies your implementation and makes maintenance and change management much easier.
Another powerful feature of Data Pages in Pega 7 is that you can define multiple data sources for the same Data Page thus allowing Pega the flexibility to dynamically choose the best Data Source at run time based on the state of the case that is using the Data Page.
The dropdown control will use the entire list, whether its data page, report definition, or page list property, to populate the dropdown. If I understand your post correctly, the best way to do what you are trying to do is to pass the desired parameter as a second parameter, so the data page populates with something similiar to SELECT * FROM <table name> where <existing filter> AND <identifier>. After doing that, when you configure the control, you should see a second param to pass, which will allow you to do what you are trying to do.