A rough rule of thumb (that works for the 767/747/380), is that any additional fuel will be totally burnt in 24 hours. So, stick an extra 1,000 kgs on, and on a 15 hour flight, 625kgs will disappear, just to carry that additional weight.
So, assuming the same rough relationship on a smaller aircraft, on a 90 minute flight, if every passenger carried 2kgs less, and you've got 150 passengers, then potentially you'd save 150*2 kgs * (1.5/24) = 18.75 kgs of fuel. At .7 kgs/litre, that's 26.7 litres. On an individual flight, it's trivia, but on a fleet of aircraft, with reasonable usage, it turns into millions over a year.
Other things come into play too. On the 94, you are almost always up against the maximum structural weight at take off. So, it doesn't matter what the requirements are at the other end, you can't accommodate them by putting more fuel on. The choices are then between taking load off (and replacing it with fuel), or living with what you have, and planning for a diversion, whilst hoping the weather improves enough to make it unnecessary.