TLDR imperialism. Israel helps the US control the middle east and also does stuff elsewhere.

Israel is basically the US's attack dog in the middle east. Any job that the US is unable to do, or would look too imperialistic to do, Israel carries out on its behalf.

Israel has played a crucial role in attacking and destabilizing independent Arab governments determined to defy US dictates. In 1967 Israel attacked two such governments in Syria and Egypt in a surprise invasion. Israel's narrative has always been that they launched a preemptive strike to prevent an impending Arab genocide of Jews, but in reality Israel just wanted to take Arab land. Mattiyahu Peled, one of Israel's commanders in the six day war, told the Israeli newspaper Haaretz,

'The thesis that that the danger of genocide was hanging over us in 1967 and that Israel was fighting for its physical existence is only a bluff, which was born and developed after the war.'

This invasion was crucial at a time when 550,000 American troops were bogged down in Vietnam fighting the spread of socialism there. The library of Congress study of the Israeli War recounted, quote,

The traumatic defeat of the Syrians and Egyptians in the June 1967 war with Israel discredited the radical socialist regimes of Nasser's Egypt and Baathist Syria. The defeat strengthened the hands of the moderates and the rightists.

In 1981, Israel bombed an Iraqi nuclear power plant with US approval at a time when the US was publicly supporting Iraq in the Iran-Iraq war. Secretly the US wanted both governments to destroy each other but couldn't be seen as playing such an intentionally destabilizing role.

in 1982, Israel invaded Lebanon with the full support and funding from the United States in an attempt to crush the Palestinian resistance there and appoint an ultra-rightwing Lebanese Phalangist as president. The US would end up sending marines in to aid this effort, only to be defeated by the Lebanese resistance forces.

Israel also has a long history of doing the empire's bidding outside the Middle East. Israel continued to arm and support apartheid South Africa even after it became too unpopular for the US to do so. Israel even built an electrified fence around Namibia, then a South African colony, to protect it from neighboring Angolan liberation forces.

Israeli police helped train torturers in the US-backed Pinochet dictatorship in Chile. They trained the right-wing Guatemalan army that was committing genocide against indigenous people. Israel has also been a loyal partner in the US war on terror. The islamophobic hysteria following 9/11 gave the US an excuse to attack any country that stood against its interests in the region, especially Iran. While the US has been careful not to directly strike Iran, Israel has served a very useful purpose by continuing to bully and harass Iran, assassinating its nuclear scientists, seizing Iranian ships, and providing intelligence on Iran through massive spy networks.

Israel also routinely bombed Syria, a country the US attempted to topple in 2011. Just by being there, Israel gives the US a constant excuse to intervene in the region under the guise of protecting Israel. As Joe Biden once put it:

Were there not an Israel, the United States of America would have to invent an Israel, to protect her interest in the region.