While Marvel Studios' love-in leading into Avengers: Infinity War has obviously been aimed at celebrating the first ten years of the MCU (with lovingly crafted recap video packages and retrospective-looking trailers), it's also served a valuable purpose for the film-makers. It's basically allowed them to run a recap on the important things we need to remember for the long-seeded event movie.
This sort of movie is pretty unprecedented, with almost an entire decade of establishing work in 18 other movies, both in terms of huge, obvious narrative swatches and in tiny, easy-to-miss details. Basically, if you haven't already, it probably makes a lot of sense to go and rewatch all of the MCU movies in the final days before the global release of the hotly-anticipated ensemble. Maybe even take notes.
But even more importantly, there are things the studio legitimately need us all to forget too: dropped balls, narrative cul-de-sacs, source material details and even entire characters in some cases who simply don't fit the narrative any more. Sometimes, those details have been retconned out of the franchise entirely, but that doesn't mean fans won't immediately think back to when they were relevant and get all distracted.
Luckily though, Marvel have done their best to either bury those details or make sure we all know that they're no longer important for Infinity War. So you just pay attention, okay?