Most classes that implement IDisposable will also clean up their resources in their finalizer. That doesn't help if it's something which you want to keep open for as small a time as possible, such as a database connection or file. It's nice that the finalizer will close the database connection eventually, but until the GC runs your table/record/whatever will be locked.

If a class has a Close or Dispose method, always call it, even if the finalizer will clean up the resources as well.