Use of buses in SONAR/Effects Sends

What are buses used for in SONAR? In other threads, it seems like they are primarily used for organization. Like all drum parts go to one bus , the guitar to naother, all harmony vocals to one bus. Is there any audio reason for using them?
What about effect sends? I usually record audio and then insert reverb and other effects into the audio track. What is the effect of using an effect send?