Structure

Structures are static constructs. They do nothing, but unlike Sky Fortresses, they don't require power to do it!.
"It", again, being nothing. Structures do nothing.

Seriously though

Structures are the simplest constructs in the game, both computationally, and in terms of their overall construction.

They have no inherent power requirements, their operational altitude is limited only by the terrain they're standing on, and they cannot be flipped over. The caveat is that once they've been built in a given location, the only way to make them not be in that exact spot anymore, is to destroy them.

Uses

• Default Designer spawn. Many players use a Sky Fortress as a platform for designing and testing various subsystems, but fortresses use power, and they use more power the further their centre of mass is from sea level. As such, a platform may start to fall apart around you if you added too much stuff to it and forgot to check how much power the fortress' turbines were using, or the AI might automatically adjust the fortress' altitude, causing it to sink into the sea, or rise into the sky, depending on whether the extra weight was added to the top or bottom of the fortress. A Structure-based test platform, by contrast, doesn't care how much weight is added to it, or where, and will always spawn in the same location, at the same altitude.
• Defending logistical infrastructure. Static defenses are generally frowned upon throughout the playerbase, but the fact of the matter is that if you've got a resource-gathering installation or major supply dump to defend, and you know roughly where the enemy's coming from, and roughly which paths they'll take, there's no reason not to erect a couple of obstacles in their way.
• Resource Gathering. It's not like the resource deposits are going anywhere either. Sky Fortresses are generally preferred for gathering resources from offshore deposits, but they constantly use power just to sit there, and nothing's stopping you from building a tower from the seabed up to the surface to keep your resource gatherers from short-circuiting.
• Resource stockpiling. Resource gathering structures should always have their own onboard stockpiles, but it also makes sense to maintain additional resource dumps closer to where the resources are going to be used.
• Decoration. The NPC factions love to do this.
• Naval mines. Technically a form of static defense, Naval mines are a breed special enough to have its own bullet point. They operate on the basis that the enemy's weakest point will be their belly, and typically come in the form of seabed torpdo batteries, magnetic mine launchers, short-ranged CRAM cannons, and the very elusive and economically questionable "Tactical Nuke on a chain".

Emplacement

1. Spawn the Structure as a Dead Blueprint.
2. Select the dead blueprint.
3. Once you've decided where to put it, place the mouse cursor over that spot, and hold down the right mouse button (RMB).
4. While holding RMB, drag the mouse around to set which direction you want the structure to point its front towards.
5. Release RMB.
6. In the Strategic view (accessed with the M key), select "Start repairs on the selected construct", just as on a regular dead blueprint.