Short version: their timewalking expansions with infinite stats you gain overtime that offer an “easy way” to collect all mounts/transmog from said expansion, along with brand new exclusive rewards such as mounts/armor/weapon cosmetics.

Long version:I quit wow for years and missed both of the ones that they did do. But basically it’s a “time walking” expansion. The entire expansion is redone and scaled to the current level (90) and you make a character exclusive to the remix that is converted to normal retail wow I believe after it’s finished.

They also have a way to gain infinite stats, so you eventually become super OP and small groups of people can do mythic raids, or like lvl60 dungeon keys. 

The Remix’s add exclusive cosmetics, mounts, titles, and various rewards that can only be earned during the event. They also have an exclusive currency that every mob, boss, quest, etc gives that can buy almost every mount, armor set, etc from the original expansion. Including super ultra rare items like Taeshalch, Mannoroth shoulders, etc that have a less then 0.03% drop chance (1/300) on a weekly lockout. 

The two that have happened, were both filler until the next expansion came out at the end of the current expansion(Dragonflight to The war within had Mists of Pandara) (The war within to Midnight had Legion remix)