Pressure groups
A representative wants to be reelected and wants to properly represent what his/her constituents want, and lobbying helps to show the representative what the voters want. By calling and sending in letters, voters can show their representatives that a certain policy has a lot of support or disapproval, and then the representative is more likely to vote in a way that represents their interests.
The Pendleton Act stipulated that government jobs should be awarded on the basis of merit. Among other things, it established competitive exams for these jobs instead of political ties being the way to obtain such jobs. An important historical result was that the parties shifted to relying on businesses for funding and not patronage from people hoping to secure government jobs.
It is called Gerrymandering.