Here's an example of what I mean:
Mark sighed and tugged at his coat collar. A cold wind blew across the ship’s bow and he could feel it shivered through his thin jacket. The sound of a foghorn made him look up. He could see saw San Francisco in the distance, its hazy lights overlooking the Bay.
In the above example, could serves to distance the reader from the verb. It inserts a buffer between the subject and the action. Mark could feel the cold, but that doesn't actually say he felt it. Just say he felt it or, better yet, show him feeling it. Mark could see San Francisco . . . but it's more vivid to say he saw it.
As words go, could is a jack-of-all-trades. This page explains it well. Could is a remnant of the English language's lost subjunctive tense and often serves to convey possibility or hypothetical situations. Hell, I could write a better romance than that piece of dreck! It also makes verbs conditional. We could go to Disney World if we get a big tax refund.
The upshot of all this? I now red flag could when revising. Sometimes I let it stand. It's a good word! Often, though, I see an opportunity to make my writing tighter by showing the action I'm saying could happen.