Whales in crypto market are making market up and down due to their high investment and also lot of coin they have. The users with huge amount of steem power can upvote the answer with good reward and for that they get curation reward from steemit. So people with huge steem power is good whales.

The purpose of the whales is to make a market green and so people enters in the market they suddenly leave and market crashed and after that they enter in new coin and make a huge profit and they do daily one trade and earned good amount of Bitcoin and so they are said to be whales.