This option, which is an opt-in agreement, must be set by organizations that use IE11 as their browser and are going to switch or have already switched to Lightning Experience.
There is no need to set this option if your organization is using Salesforce Classic and not the Lightning Experience interface.
At the time of writing, IE11 is a supported browser for Salesforce Classic organizations. Lightning Experience organizations can set this option for IE11 extended browser support until December 2020.
The following section talks about caching.
