Why Egypt is so important to the United States

Why is Egypt important to United States? The country has been America's closest ally in the Arab world.
For years, Washington has been a big supporter of the Mubarak regime giving it billions of dollars in military and economic aid.The big fear in Washington is that if Mubarak goes, then groups hostile to America like the Muslim Brotherhood could take charge.The Muslim Brotherhood is the biggest opposition and Islamic group in Egypt, their goal is to create a state ruled by sharia law.