This is kind of a blurry question, and the answers are going to be somewhat speculative.
The amalgamated questions are:
- What are structures intended to be for?
- What are structures objectively for?
- What motivates players to own structures?
As covered by some of the previous posters, structures were intended to be a replacement of the old Player Owned Station/Starbase, the control towers and associated goodies you’d anchor around it, and the outposts/conquerable stations in sov null that were converted to faction fortizars. The hype was that these would be simple to set up. Rigs were intended to be a replacement for outpost upgrades, which is why they are expensive. (However, outposts were limited to one per system and only in systems that you could attain sov.) Structures were also intended to be used as industrial hubs, and safety was high (meaning asset safety, the vulnerability timers, etc) to encourage adoption and use by capsuleers throughout new eden who would otherwise be afraid of losing all their stuff. The new upwell structures competed with outposts (and NPC stations to some degree), which were indestructible. Outposts could change hands and you could lose access, but your stuff would still be there and could potentially be recovered in the future. While I do recall CCP showing us we could tax people using our stations, I do not believe it was a thing they focused on. You can, if you can get people to use your station, possibly make some isk.
Structures are more objectively for war control and moon mining. Both are very binary. You either have an athanor on a moon and can frack it, or you completely can’t. You either have a structure and can wardec, or you don’t. The war vulnerability and war HQ mechanics were not part of the original vision that we were told, but came about as part of a push to establish ‘social corporations’ that were immune to war, like NPC corporations were. You can save money in taxes if you do a lot of industry or the tax in your system is really high, or you can try to open the structure to the public and collect tax that way. The latter doesn’t work well because capsuleers think other capsuleers are a dodgy unreliable bunch and would rather use their own structure.
Like the corporations of yore that were something of a milestone in one’s eve career, owning a structures is such a milestone, and desirable on that merit alone, even if it is not useful. Flying your own flag and having your own presence in space that is uniquely yours is something a player can feel proud of, even if there’s no practical gain (or even a loss). Some people need access to services that are not provided for in certain systems, or access to cloning facilities they can be confident of. Unlike assets, clone bays shut down deny access to the clones stored there, which could have expensive implants. If the clone bay is removed, those clones are destroyed. Furthermore, jobs can be halted or lost, and a structure under attack and offlined does not generate warnings for users in all cases, but will alert the owner, so ownership prevents you from being broadsided by an alert that your stuff is already gone. My opinion is that it’s a lack of trust and the difficulty of overcoming the interaction barrier that coupled with the view of it as an achievement that has so many of them put in space. Offline structures remain in space because it’s difficult to take them down, and a majority of the value of a rigged structure is in the rigs that you can’t recover or resell. (They also can’t drop, which hurts their value in so far as motivating an attacker to decommission the structure.)
Personally, I use structures as a means to establish a presence and interact. I do not make money, but by hanging around for a while and building up a prerequisite trust I do get some people to use mine and save them some isk in fuel costs as opposed to running my own. In turn, I borrow others structures for tasks/services outside my specialty, and pay a small tax instead of a huge fuel expense. I like knowing to whom my money goes, and I am willing to be inconvenienced to some degree and use local services with greater risk/less efficiency.
Announcing yourself in some systems can generate a bit of conflict. Lowsec has some folks who’ll reach out to you if you establish a base. Some might ask to use it, and some might demand isk from you not to destroy it, but whichever one it is, it generates some response, if poking the hornet’s nest is a thing you find interesting.