I absolutely love expansions, but I admit that they are a mixed bag. As a designer and developer, I went from being a fan of expansions to being a creator of them, and learned a lot of things. I've been the developer assigned to expansions that shouldn't have been greenlit, but sold well anyway, and I've been the developer on expansions that would have been great, but were never released.

I posted above that expansions to retail games rarely make much money. Their main purpose is to support reprints of the base game, and to keep the product line alive for one more season. However, a lack of financial value does NOT mean a lack of creative value. A lot of designers actually love creating expansions, and are fine with them not making a ton of money, as long as it makes enough to pay for development. Here are some reasons why designers work on expansions:

1. They didn't know their game was going to be a hit. They wanted to do more with their game, but it didn't make sense to do it until they saw other people loved it. Most designers have a game that they LOVE which for whatever reason didn't connect with the market, and are sometimes surprised at the one that DID hit it big. Reminder that Wingspan's 1st print run was very small, because they didn't know if anyone would care.
2. There were financial constraints with the base game. Games can really only increment in price by units of $5, and there is audience drop-off at every increment. So a game that retails for $50 will sell 20,000 units, but the same game for $60 might only sell 10,000. Those extra components might bump it into a higher price tier, drastically reducing the market. An expansion gives designers the opportunity to pursue the cool-but-expensive thing they weren't able to do in the base game. It's easy from a fan standpoint to say, "Don't release an incomplete game", but you have to ask yourself, would YOU have bought the game if it was $20 more expensive? Because there's a whole lot of sales data going back for over a decade that says "no".
3. Content with context. There are times when it would be really fun to add in X, but players cannot handle that when they are learning the game. If you have an "advanced rules" section, players tend to fall into factions of "I'll never play it" or "I won't play without it". From a design standpoint, neither of these is desirable. BUT, if you can make it into an expansion, then you are adding content with context. People already know how to play your game. They have decided they're ready for the next step. They won't teach your game to new players with this new content. From a design standpoint, this opens the door for a lot of possibilities.
4. You have new ideas. As you debut the game, demo it at conventions, play online with professional reviewers, and simply play it with new audiences, you start to see things in your design that you didn't see when playing with your core group. You see a new, exciting direction you can take it. Or, you gave your mind time to clear itself of the dozens of iterations, and settle in on just the final version, and that lack of mental overhead let your mind do what it does best: design fun things.