It does have significance in that she carries with her a near infinite amount of investment from the playerbase compared to the Jailer. Defeating her will be cathartic and feel like a major ending to a story that has technically been going since Warcraft 3. Or 2 expansions if you only count the ones where her motivations are starting to become clear.
Sure, the Jailer can be cool and all, and i have no doubt that the expansion will give us reasons to hate him. But why should the average player care about him when we just defeated Sylvanas, the character that several expansion have revolved around defeating. It would at best be only a slight downgrade to fight the Jailer unless he literally ressurects a whole bunch of former WoW villains to carry emotional investment in his stead.