What is the reason for killing whales?

They shouldn't. The human race gains absolutely no benefit from killing whales.

Whales are killed for their bones, which is used to carve wares, and for their flesh, used in (mostly) Asian dishes.

Many whales are protected species, so killing them is a violation of the law. Unfortunately this does not stop people hunting them. The largest culprit of whaling is Japan. International pressure is forcing whaling countries to stop or gradually stop whaling practices. Recently, Iceland was rejected membership to the European Union purely because of its whaling operations. Europe does not want to be associated with such inhuman acts.

Some countries, such as Chile, actually have a portion of their navy patrolling the oceans which are habitats to protected whales, they are given permission to apprehend or engage any ship that attacks a whale.

Whale hunting is highly illegal.