Agents work with a lot of similar tickets day in and day out. Tickets that require them to perform, pretty much, the same actions again and again and again. For example, after a major update, many customers report the same bug and agents will have to send the exact same response to everybody before closing the ticket.
Scenarios make these repetitive tasks a whole lot easier by letting the agents perform multiple tasks with a single click. So for the above example, an agent can create a scenario called 'Bug report' and add actions (sending email and closing ticket) under it. This way, the agent can select all the tickets perform the scenario using bulk actions or execute scenario for individual tickets.