This function returns the name of the issue type with the given id.
issueType(id) #Output: Text
You might want to use this function for several parser expression where you are e.g. checking against the current issue type. So even if you rename the issue type afterwards, the parser expression still works and won't break.
Imagine an expression like this:
%{issue.issueType} = issueType(10000)
instead of:
%{issue.issueType} = "Task"