Well if a member of the Senate decides that they are dedicated to a bill but it has already been decided that it will be killed, they can take the floor and talk, and talk, and talk, and talk, to delay the vote to kill the bill. This way, they can get their ideas out about why the bill should live AND no one else can take the floor from them to continue the process. Sometimes after a filibuster takes place, the Senate will change its mind, but most times it is simply a time waster.