Adventure films like Star Wars may have say three battles in a day, maybe four -
Star Wars has two firefights in the Death Star prison block, the attack of the garbage
monster, then Luke has a shoot-out with stormtroopers on the way out.
They long rest on the way to Alderran.
They then get picked up by the tractor beam, take out some stormtroopers on the MF (before putting on their armor) wander down to the detention block, take out the guards, get into another fight on the way out, the trash monster then nearly kills Luke, the compactor nearly kills them all, then there is a running skirmish on the way out with Han (and Luke and Leia get into another skirmish over the chasm they swing over). A running gun battle then takes place as they escape, followed by a spaceship battle once they leave the death star.
They then get a short rest in on the way to Yavin (to spend some hit dice), before Luke has to take on the imperial fleet and blow up the death star.
Thats a pretty full on adventuring day!
my own reading of the 5e DMG
was that this was the number of fights that would exhaust PC resources, not a number required for balance
purposes.
Thats just a different way of saying the same thing.
If you only have 2 fights per long rest, barbarians effectively have double the HP of fighters, a permament +2 to damage, and advantage on str checks (from 1st level). Raging barbarians have the edge over Fighters (as do Paladins if they are allowed to smite nova).
By forcing the barbarians and paladins to stretch those long rest resources over 6-8 encounters (and 2 short rests) the classes balance out.
Try running 6 encounters with a Warlock 9 and a Wizard 9 (no short rests). Your poor 'lock has 2 spells to last him 6 encounters (30 rounds on average). Your Wizard has 15. Now insert 2 shor rests in there, and youll see it balances out (the 'lock has 6 slots of 5th level (or 1 per encounter) to the Wizards 2 5ths, 3 4ths and 3rds, 4 2nds and 1sts. Yeah the Wizard has more spells, but the locks fewer spells come out at max level, and he has at will invocations and better at will cantrip damage to fall back on).
Not all classes are as dependent on short rests as others or as dependent on long rests as others. You give more short rests, and certain classes gain more advantage from that. You push fewer encounters on the classes and those with long rest dependent abilities (spells mainly) which are more potent than short rest powers, gain the biggest benefit.
Imagine a 6 encounter adventuring day (6 encounters between long rests). Now throw 1 short rest after every encounter. Warlocks, monks and fighters will rise to the top spamming action surge, second wind, ki, stunning fist, spells and superioirty dice like candy. Now do the same thing again with no short rests between the 6 encounters and those classes fall away and suck badly.
The sweet spot is 6-8 medium to hard encounters and around 2 short rests. I aim for 6 hard encounters myself personally, with a short rest available every 2 or so encounters.