No, don't have to. Checking in at the airports fine too. but if you do check in online you usually get the opportunity to choose a seat, pay any applicable checked bag fees and consider a variety of upcharges (such as an upgrade to premium economy or a fee to double your frequent flier miles). You may also have to answer some security questions or supply a passport number for an international flight.
Yes you check in at airport, just get there 30 mints before normal time
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