Please read the wiki. Combo is not DIRECTLY used, but there is something similar to combo that is part of the calculation.
Score is calculated in 2 parts, one part based 100% on accuracy, and one part that uses accuracy and a multiplier. This special multiplier starts at maximum, but if you hit anything less than 300, it will be reduced. If you miss, it is reduced to 0. You can think of the multiplier like HP, it starts out max, but if you make a mistake, it drops and you need to build it back up.
Because the value of this bonus is carried between hits, any time you make a hit that is not a 300 or max, you will not be getting the maximum possible points for that note AND notes after it, until you build this bonus back up to maximum.
Without getting into the exact math, the effect is this:
If you miss a few times spread out across the map, that will make your score worse than if you missed the same number of times all in a row. That means that When you missed matters, and can cause 2 scores with identical stats (same number of notes for each judgement) to end up with different final scores.