They're surrounded by enemies in the Middle East, and they need every bit of national security information they can get. Well, actually, we know from past experience that the Israelis not only steal our secrets, they sell these secrets to our enemies. Selling U.S. military and diplomatic secrets to the Soviet Union back during the Cold War was standard operating procedure for the Israelis. These days they have a willing buyer for stolen secrets in the form of China. Beyond selling secrets, Israeli spying has a much more sinister purpose, as former Central Intelligence Agency Director William Casey indicated to Thomas in an interview.
Most countries spy for security purposes.
Israel spies in order to control the country on which she is spying, primarily through the blackmail of politicians. In the case of the Lewinsky telephone taps there are strong indications that the Israelis used the information they collected to blackmail Clinton. They wanted him to halt an FBI search for an Israeli agent on the White House staff, otherwise they would leak their recordings of his telephone-sex with Monica to the press. The White House denies there was any blackmail, and of course the Israelis deny it. Gordon Thomas says that he has no evidence of direct Israeli blackmail and suggests that it probably was more subtle and indirect than that.