Jelly is not reserved for building custom pages. Since the ServiceNow platform is built using the language, there are several places you can inject your own code to further control your instance.
One of the simplest things to do with Jelly is to create a formatter. This provides a mechanism to include a UI Macro in a form. Navigate to System UI > Formatters to see the out-of-the-box items. There are several examples, including the process flow formatter, the activity log, and the variable editor placed on the ITSM tables, that show what is possible.
Let's cover a simple but useful example. This will run on the Maintenance form, making it more obvious when an SLA has been breached: