Members of Congress use these techniques because they can, and it is in their interest to do so. The voting sessions contain a small amount of parties which allows the possibility that members can purchase others members' votes.
The most important aspect here is that these votes are kept on public record so it is easy to verify whether someone went through with the agreement. If one member discovers the other did the opposite of what they agreed, they would be ostracized and would have little bargaining power to pass their own measures they are pushing.
logrolling
Logrolling
Logrolling
Logrolling
Logrolling
Logrolling
Logrolling
Logrolling
logrolling
Members of Congress often support each other based on a simple promise of "I'll vote for 'x' if you vote for 'y.'" This practice of making these informal promises of support is called logrolling. Party leaders and whips keep track of vote debts between party members and use them to gain support when important party-sponsored legislation must be passed.
It's the act of rolling a log upon a surface of matter.
Committee staff may influence congress by granting or withholding legislation. Organizations can also influences members of congress in the same ways.