Why must they destroy the buildings which exist there now as they pull out? Sems a waste to leave only rubble behind..perhaps they could leave the buildings intact so that the Palestinians are not inheriting a mess?

What you say is true, Wolfgang, but not important.

Israel is withdrawing from Gaza, which would have to happen at some point.

Israel can avoid having Palestinians occupy buildings which people still see as their homes, and flying Palestinian Flags over their synogogues>. Any pictures of that would be like those of the Berlin Wall coming down, or the last helicopter leaving Vietnam, a powerful image summing up a great victory/defeat.

This is why the buildings are coming down. Occupying a patch of rubble may be significant, but it's not iconic.