system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR] What is the purpose of Sauron's tower/eye thing in Mordor?

He can only see a small area in his line of sight? What's the point of that? Is he trapped? Surely someone as powerful as he is can have physical form.
gpt
In the books, the "Eye of Sauron" was a metaphor for Sauron's influence, not a literal eye. In the books Sauron had a body. 

In the movies, they decided that the threat of the Ring was more real or easier to drive home if they suggested that Sauron can't "take physical form" without the Ring. (I believe they had Saruman say this.) It simplified the Ring's importance to the audience, and eliminates the inevitable questions that would arise from having your primary villain sit inside his tower for the entire trilogy. Making the eye a visual motif was something they determined to be more menacing than a dude in a tower, and more appropriate for the medium of film.

Having an unseen villain works better in a book than it does in an inherently visual medium, so one can see why they wanted to go in that direction to make him more compelling to audiences. But understand that "the Eye" was a solution to a practical problem in translating between mediums, and as such it shakes up the canon and creates questions like yours.

If we take the movies to be the canon in question and ignore the books: then the Eye is the spiritual manifestation of Sauron, kept alive presumably by the dark magic of the tower, until the Ring can be obtained to stabilize his existence permanently in this world.