Congressional leaders form committees when they want to take up an issue that falls between the jurisdictions of existing committees, to high-light an issue, or to investigate a particular problem.

Congressional leaders form select committees when they want to take up issues that fall outside the jurisdictions of existing committees, to highlight an issue, or to investigate a particular problem.

Explanation:

Select committees conduct hearings and function as focal points for the issues they are assigned for considering.  

These committees are granted the right to sit at any time and anywhere in the United States, whether or not Congress is in session. They have the right to call witnesses and demand the provision of necessary documentation. Committee hearings are usually held openly. For refusing to answer the questions of the committee members, the persons summoned to the hearings can be fined up to $1,000 or imprisoned for up to one year. The reports of the special and investigation committees are heard by the chambers that formed them. After hearing reports, special committees are usually disbanded.

political parties are protected under 6th amendment.