Sometimes airlines are unable, for any number of reasons, to fly to the destination printed on the ticket. If your flight is diverted, the airline must get you to the destination airport on your reservation* – at no extra cost to you.
Sometimes the airline may arrange buses or taxis. Or it may ask you to make your own transport arrangements and send in the receipt to be reimbursed.
The important point is that it should, in the end, not cost you any extra to get to your destination. However, if the airline arranges a bus and you choose to go by other means you will struggle to get the airline to reimburse you.
*The main exception to this would be if a flight was disrupted to avoid war or civil strife.