Controller accounts are deprecated. For more information, see this discussion.
Controller accounts were used for staking and were a "less-powerful" version of staking proxies. Controllers could only sign for unbonding and rebonding funds, nominating and changing the reward destination. The stash account was still used to bond more funds and change the controller. Controller accounts became redundant and added unnecessary complexity to the staking mechanics.
With the setup shown above, the stash account was not entirely isolated. More complicated designs to fully isolate the stash account included having both controller and staking proxies (see below).
Stash as Controller
It was unnecessary to have a controller if you had a staking proxy. In this case the stash was also set to be the controller, and the account security would not have been compromised. The staking proxy was used to sign all staking-relate transactions. Note that you needed to sign with the stash to change the staking proxy.
This past situation was similar to the present setup, except that now there is no option to set the stash as controller and that the action of "changing the controller" is missing. From a practical perspective, we need to use only one account and remember one password to sign for all staking-related transactions. From a security perspective, who controls the staking proxy controls our staking actions.
Stash not as Controller
If the stash and controller were different accounts, the staking proxy was used to bond more funds and change the controller. Thus the staking proxy was used to sign for those transactions that were used less often and usually signed by the stash.
From a practical perspective, there were two accounts, and we needed to remember two passwords. From a security perspective, the party who wanted to control our staking actions was required to control two accounts.