I'm talking about the flares you see at oil refineries, not drilling sites. Those are pressure relief valves and are essential safety equipment.
The flares you see on a day-to-day basis are just pilot flames which are always kept lit in the event a flare is required during an emergency such as an over-pressurization.
The pressure relief values are essential, what happens after that point is a separate issue. The advantage of Flares is there simple, cheap, can quickly deal with a lot of gas, and render the gas inert. Holding tanks on the other hand need a lot of piping, pumping equipment, and then some system to deal with whatever miscellaneous gas ends up in there. However, if C02 start being taxed expect to see far fewer flares.
The flares you see on a day-to-day basis are just pilot flames which are always kept lit in the event a flare is required during an emergency such as an over-pressurization.