The APN enable/disable setting facilitates choosing which APN configuration of multiple choices should be used for general internet data.
If you have multiple APN configurations that support general internet data (that is, with an APN type including "default" or the catch-all "*") then this setting controls which one shall be used. The setting is mutually exclusive, so selecting "enable" here will disable other relevant configurations.
If you only have one APN configuration that supports internet data, or if this isn't one of them, changing this setting isn't possible and this field will probably be disabled.
Note: Checking this box is equivalent to selecting an APN configuration by tapping next to it in the menu in the "APNs" screen.