Possibly because people use wallets because we prefer our bitcoins to be safe therefore using offline ones like armory and such...and there is a need for speed and fast transactions sometimes therefore using other wallets like blockchain/electrum and such.
well yeah i really like to use a lot of bitcoin wallets in my opinion because it allows us to keep bitcoins safe
Not only for bitcoin safe they are using many wallets to secured their original wallet which is main wallet where you can see all of your savings..
They are using many wallets to use for easy transaction and fast secured. they can also use bitcoin mixer but they have their own way to secured their funds.