Regarding query-1, Tickets are generally used when you want to jump your flow to a specific shape of a child class. Alternately, you can use Use Alternate Stage. Refer the post .
Regarding query-2, as per PDN help of Pega 7.3, You can use a ticket in a flow to respond to business exceptions, errors, or events that you define. By moving a case to a specific point in a flow, you can ensure that additional processing occurs so that the case is not left in an incomplete state.
For example using tickets, you can activate a ticket to perform cancellation tasks and send correspondence to stakeholders when a user withdraws a case.