The FBI went after Page as if he were a real spy or terrorist. As you've no doubt seen in countless police dramas, law enforcement wants to know all about the phone calls, emails and texts the target sends to his/her associates. They also want to know who those associates are communicating with as well. The spying goes out what's called "2-Hops" from the target.
With the spying on Carter Page, it meant that the FBI spied on his communications with Senior Trump advisor Steve Bannon who was in contact with candidate Donald Trump. The result: the FBI could snoop on all of their communications.