What are Ice Towers useful for?

The immunity to sharp objects does not extend to explosive damage. Thus, you could combine an Ice Tower with a Bomb Tower, Mortar Tower, Monkey Apprentice, Super Monkey, or simply a Pineapple to freeze bloons in place while destroying them.

The Ice Tower also has upgrades that allow it to pop bloons as they're frozen, slow down bloons even after defrosting, and more.

Other towers also have upgrades that will allow them to pop frozen bloons, such as the Sniper Monkey, the Boomerang Thrower, and the Dartling Gun. Additionally, the Monkey Village can be upgraded to permit all towers within its range to pop any type of bloon, including frozen bloons.

In short, Ice Towers are capable of holding bloons in place for other towers to shoot at, as well as slowing down waves.