Ways to plot dendrograms in R

I know the purpose of this is to just demonstrate/present the code necessary to plot dendrograms but a good complimentary blog post would discuss the useful of actually executing that code:

-When for instance is it appropriate to use dendrograms (hierarchal clustering) versus non-hierarchal approaches (ie. k-means clustering)?

-What does it actually mean for two observations to be clustered close together in a dendrogram (and would they also be clustered together if you uses a non-hierarchal approach)?