For the longest time, experience points did not matter in Rocket League. They simply existed to make the cumulative numbers go higher and higher. XP wasn't really attached to anything other than a rough estimate of how long someone had spent playing the game (which was then rendered moot for anyone who hit the level cap which came at level 75).
That has drastically changed in the past month-or-so. Psyonix has removed Rocket League's level cap and reconfigured the way item drops work. Now, a random item accompanies every new level earned. It's paired with the recently-introduced Rocket Pass system which awards its own items at each new level. For anyone who cares at all about Rocket League's cosmetics, experience points matter a whole lot now. They're the sole gatekeeper to all non-crate items. It has been quite the 180-degree flip.
It finally makes sense for Psyonix to run a double XP weekend. Starting today (the event is live as of 4pm Pacific) playing Rocket League will yield twice as much XP. That directly correlates to twice as many item drops. It ends on Monday October 15 at 10am Pacific.
Rocket League has had similar events in the past, like weekends where items would drop twice as frequently or painted items would be easier to come by. It has never dealt in experience points before because experience points were never actually relevant. Now, with the recent overhaul, Psyonix can just cut to the chase and use XP as the incentive.
SARPBC's 10th Anniversary and Double XP Weekend [Rocket League]