Spell slots are a resource you spend to cast the spells that are ready to be cast. Very simple, easy to manage, and doesn't require professional-level Plot Sense to prepare for an adventure. Being able to up-cast a spell means you have a lot more spells available to you (e.g. instead of a 2nd level fire damage spell and a 3rd level fire damage spell that you have to have prepared, you just prepare the lower one, which can be cast as 3rd level).
The only thing missing is a conversion rate between spell levels. For instance, if I want to sacrifice a 5th level spell slot, how many lower level spell slots would I get? If it had that, the spell system would be functionally identical to a spell point system.