Headline statement
JQL supports querying issues out of the box, but there is often cases where more complex queries are required. Being able to write JQL functions through Forge will help bring a new level flexibility to developers and teams.
Example
A user wants to write some JQL which checks for issues where the epic link is one of the epics in a project. A developer in the team is comfortable writing Forge apps and would like to write a JQL Function epicsInProject() to complete the JQL
issuetype = Story and "Epic Link" in epicsInProject("Project X")