What is so important?

Question

What is the importance that water expands when frozen?

Answer 1

p***s emplants for frosty the snow man

Answer 2

It's one of the few molecules that becomes less dense in the phase transition from liquid to solid.

Answer 3

It is the only substance that does it.  It requires special engineering considerations when building roads, buildings, sidewalks, etc.  If it is not accounted for, the frozen water could break or buckle these things.