It’s mainly good if you do want to experience maximum player power of an expansion. I remember how weak my priest was at the start of SL then became a powerhouse with full mythic ilvl gear by when DF would launch.
Also, pretty much everything gets unlocked and the content cycling a little taking turns every week isn’t as bland as only getting to do the latest raid. Bit like raiding back in the day where every raid was relevant in a manner, if nothing else, than for the sake of gearing up for the next raid. You know, MC gear for BWL, BWL for AQ etc…
Or, if you cannot be bothered and wanna save money/play something else, this is the best time to unsub.