Post functions allow you to perform specific actions upon the transition. Following are some useful examples.

import com.atlassian.jira.component.ComponentAccessor
 
def commentManager = ComponentAccessor.getCommentManager()
def currentUser = ComponentAccessor.getJiraAuthenticationContext().getLoggedInUser()
    commentManager.create(
        issue,
        currentUser,
        "This is my comment",
        false) 

import com.atlassian.jira.component.ComponentAccessor
 
def versionManager = ComponentAccessor.getVersionManager()
def projectManager = ComponentAccessor.getProjectManager()
def project = projectManager.getProjectObjByKey(issue.projectObject.key)
def versions = versionManager.getVersions(project)
def firstUnreleasedFound = versions.find{! it.released}
 
issue.setFixVersions([firstUnreleasedFound])
issue.store()