As well known, Perelman proved Poincare conjecture by proving Thurston's Geometrization conjecture.
Somebody says that we can understand part of the universe from Poincare conjecture.
As a purely topological viewpoint, why do you think the poincare conjecutre is important and how about Smooth poincare conjecture in dimension 4?
One may simply answer that because it is extremely difficult to prove or because it made 100 years of development of history of Topology or geometry....
But I want to know your professional, new, own viewpoint.