There are three ways that committees aid Congress' functionality:
Division of labor. This aids in Congress' efficiency.
Specialization. There is value in specialists with niche knowledge.
Re-election. Members of Congress apply to their party to be on committees pertinent to their state. For example, an Iowan ought be on an agricultural committee and a San Franciscan ought be on a maritime committee.
Types of Congressional Committees:
Standing Committees (permanent)
Subcommittees (more specialized)
Select Committees (created to accomplish a specific task)
Joint Committees (contain both Representatives and Senators)
Conference Committees (resolve legislative disagreements House and Senate; legislation disagreed upon can be compromised by a conference committee)
Rules Committee (specific to the House, the rules committee establishes rules for debate once a bill makes it to the floor of the House)
Party leadership assigns Members of Congress to committees, assigning almost all their own party to each committee. However, party leadership often includes one or two members of the opposition party due to reciprocity. This way they will still be included when they lose the majority.