The role of Congressional committees in the legislative process is to act as the experts on a given topic. They use their expertise to amend proposals in their policy area.
Congress, of course, has to deal with a tremendous range of issues. They have to, for example, deal with everything from the complexities of copyright laws to the best ways to help farmers economically. It is very difficult for any member of Congress to be an expert in all of these areas.
It is also very difficult to imagine how Congress would work if all members of each house had to consider each bill on the floor. It would be impossible to get much done if, in the case of the House, there were 435 members wanting to talk about each bill. There would not be enough time to deal with all the bills Congress must address.
For these reasons, committees have been created. Committees allow some members to become experts in given policy areas. They also allow Congress to divide up the work of considering bills so that more bills can be handled each year. Thus, committees allow Congress to act more efficiently and with more expertise than would otherwise be possible.