As some folks have mentioned, there are a variey of reasons, mainly economic and operational, why airliners don't go around routinely flying with full tanks. You'd be surprised how some folks *within* the industry and ATC think that's the case, but I digress...
Various FAA regs address fuel requirements that dispatchers must consider. You need fuel to get from point-A to point-B. You need fuel to get to a viable alternate point-C if the weather at B is (or forecast to be) below a certain value. You need fuel for a :45 minute reserve (that's necer planned for use). You need fuel for any enroute weather deviations around storms, and for any known/probable ATC delays.
Depending upon the operational variables for a specfic flight, the resultant fuel is known as "required" fuel, and we strive to keep that number as low as possible. One, to leave room for payload (pax, bags, etc.) and two, to minimize the fuel consumption it'll take to fly that additional fuel around (about 2-3%).
In some cases, we also carry extra fuel (above required) for cost-differential tankering, or other operational purposes. For example, if the aircraft was going from DFW-XYZ-DFW and the cost of fuel was higher at XYZ, it might make more sense to take enough extra fuel from DFW sufficient to have enough for the return XYZ-DFW flight, without the need for buying of the more expensive fuel at XYZ. We do this all the time, if we cam, payload permitting, and applied system-wide, it can and does save oodles of $$$, especially when fuel prices vary greatly and change so often.
Sometimes, fuel trucks and fuel farms are inop for some reason or another, and we take this kind of extra fuel along, not for economic reasons, but to avoid getting stranded since no fueling is available.
Lots of variables....
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.