When giving a message to a user, commenly you want to give the user either a Positive and a Negative option; or Yes / No.
Introducing more than two buttons may overwhelm the user and could lead to them incorretly making a choice that has consequences that he/she may or may not understands.
Having the ability to add a checkbox to a message can help the user in allowing them to check the box and explaining the functionality of it. Many applications and web browsers already do it that way, one example is shown under here:

The checkbox should be mappable to a process variable when used in a Process flow - Process action Show message. This way we can obtain the state and process the chosen option accordingly.