Prior to the 1967 Six Day War, Syria had been using the Golan Heights to shell Israeli kibbutzes in Galilee. Israel gained control of the Golan Heights and Mt. Hermon in 1967 and kept it despite the 1973 Yom Kippur War. Israel has advanced a few hundred or a couple thousand meters farther into the Heights and the Mt. as extra buffer against the uncertain intent of whoever controls or will control Syria. "Indefinitely" may merely mean that when the situation stabilizes in Syria and if the new government shows a willingness to coexist with Israel, Israel will withdraw to the 2023 border.
In parallel, Israel has been busily destroying a large part of Syria's military assets, including chemical weapons. This may help persuade the future government of Syria that coexistence would be a wise choice.