If you fail to check in on time and the flight is overbooked, the airline can cancel your booking and give your seat to somebody else. The Travel agent has to confirm with the airline that your status is fine before your ticket is valid. Prior to this confirmation your status is on request. Businesses often need to send urgent documents or freight securely and quickly. May have to give all your baggage allowance for the use of the courier company and be only allowed to take carry-on luggage.
Most unused tickets are refundable.
However, legally an airline can refuse reissuing a replacement - so if you lose a ticket it could be forever. Discount ticket holders are penalized see cancellation penalties. Usually the excess passengers balance those who fail to show up. You must contact the airline at least 72 hours before departure to reconfirm that you intend to be on the flight. Otherwise the airline can delete your name from the passenger list and you could lose your seat.