Online air tickets are really cheap. Some time you may get up to 60% discount on your flight ticket and can save lot of money. The money you saved in the air ticket can be spent on your vacation trip.
You can search for the suitable date and suitable prices of the online air ticket. If you are a member in a particular air line you can buy cheap air tickets for your spouse or child to travel alone without yourself accompanying them. Your credit card company may allow you to pay the air ticket fare in a long term basis. You may get traveler insurance from the air line at considerable low rate. There is no need to sit in the travel agent office and waiting for your turn to come.
But on the other hand you have to book these tickets long time before your travel plan as they won't be available for cheaper price. The luggage weight is much reduced than the normal flights in the cheap online tickets.
Once it is booked it can't be cancelled.
Most of the air lines don't allow you to purchase an air ticket online for another person with your credit card if you are not accompanying him in the same flight.
Seat plan, food and onboard beverages should be paid by you.
On some long distance flights you have to spend one night in a hotel in the air line's main office city to get the connecting flight. (There is no hotel charge for you.)