Main Idea: Give Immortal Empires access to people who only own games 1, 2 or 3. The standalone campaigns are not that fun imo and its the best way to get them interested in the other factions too.
Further: Those people already missed out on any value they could have gotten from those games when its current anyway, so expecting them to buy it to gamble on the fact that they're going to like immortal empires is kind of silly. Obviously you should still be locked to factions you own. After the way the Realms of Chaos was, I feel we owe our brothers who bought into the hype train a small conciliatory prize , and who knows it may redeem the game entirely to them.
TL;DR- on the business side its an excellent advertisement for DLC. On the human side, I feel bad for our brothers who heard about all the fun we had in 1&2, bought in and then got....that.
Inb4 "I paid for it they should have to pay too." That's great dude we'll get you a medal and maybe a holiday. But they're getting significantly less of the experience as a trial run to see if they want to buy the rest.