Storyline wise, noone is guarenteed to be in either so it's not pointless. You're making it sound like talent has a choice between which match they want to be in, which they don't, they have to take what they can get.
It's another chance to get to that main event spot, and the losers set up there own rivalries or further them going into Mania as has been said
and there's a difference, one is a guarenteed shot at the title for only one person in the company, like the MITB briefcase
the other is a chance at winning the title on a subpar PPV and the last PPV chance to get to the WrestleMania main title match.
Meanwhile the winner of the Rumble, even if they lose E.C are still the number 1 contender unless that stipulation is in place, which it hasn't been to date.
It's another chance to get to that main event spot, and the losers set up there own rivalries or further them going into Mania as has been said
and there's a difference, one is a guarenteed shot at the title for only one person in the company, like the MITB briefcase
the other is a chance at winning the title on a subpar PPV and the last PPV chance to get to the WrestleMania main title match.
Meanwhile the winner of the Rumble, even if they lose E.C are still the number 1 contender unless that stipulation is in place, which it hasn't been to date.