When the "disengagement" from Gaza was carried out two years ago, there were many claims about how it was going to help Israel's security. All of them, of course, turned out to be delusional.
There was also a claim from the Palestinian side about how the disengagement was really a conspiracy to somehow "deepen the occupation" in the West Bank. Israelis of course ridiculed this claim, because the withdrawal included part of the West Bank, and because the Israeli government is too confused and discordant to carry out sophisticated conspiracies.
But in retrospect, the Palestinians were right. In the last few months, and most dramatically in the last few days, we have seen the emergence of one significant and tangible benefit from disengagement. It is unintended, but it is real. This is the separation of Gaza from the West Bank.
The total withdrawal from Gaza, accompanied by Israel's relative control over the West Bank, allowed the two to go in different political directions. In the West Bank, where Israel has security control, the competition between Fatah and Hamas is mostly economic and social. Meanwhile, in Gaza the competition is military. This has quite logically allowed Fatah to remain in control of the West Bank (due to its political primacy and Western funding), while Hamas has conquered Gaza (thanks to its military advantage).
The establishment of two separate governments is in Israel's interest. In the short term, it may allow Israel to pursue policies in Gaza unencumbered by the political situation in the West Bank, and vice versa. It will lessen the chances of Kassam rockets and other such weapons reaching the West Bank. And it will provide some international legitimacy for harsher action against Hamas in Gaza, since it will be clear that Israel is not attacking "the Palestinians", but rather a particular organization with which it is at at war.
The long-term benefits are much greater. West Bank residents will want less and less to do with Gaza as it self-destructs. Already they have begun to direct their political aspirations eastward, towards union with Jordan, and not west towards Israel and Gaza. The Gazan civil war have shaken their confidence in likelihood of building a stable Palestinian society anywhere.
Even more significantly, the fundamental assumption of Palestinian peoplehood is being challenged. This is extremely important, because Palestinian and Israeli nationalism are to a large extent mutually exclusive. If you look at "Palestine" on a map (West Bank and Gaza), you immediately notice the big space in the middle - Israel. You realize that "Palestine"'s current borders are unnatural, than all of Israel "should" have been part of Palestine, and would have been, were it not for Jewish conquests in 1948. If you sympathize with "Palestine", you will unwittingly or wittingly speculate about somehow reattaching Israel to "Palestine", thus forming natural and reasonable boundaries for the Palestinian state. Thus, the existence of a Palestinian collective is a constant challenge to Israel's legitimacy. This challenge is decades-old, heavily exploited in both Arab countries and the West, but it appears to be ending. What remains are two separate boundary disputes, one east of Israel and one to the west. And the chance of resolving each of these individually (by whatever solution you prefer) is much higher.