Commercial ops require that you arrive over your destination with:
1. Fuel to required alternate(s) or specific extra fuel in leu of alternate (Hawaii, Guam, etc)
2. Reserve fuel specified by regulations (30 or 45 mins)
3. Company required extra
4. Local regulations (ie: Singapore requires "60 minutes over SIN", where regardless of where your alternate may be, you must have 60 minutes of fuel in the tanks when arriving over the airport)
Of course, Captains are paid to make good decisions, and therefore may choose to carry additional fuel for whatever friggin reasons that we can come up with.