-くる shows progress or "movement", so when combined with 見える it is the "in the progress of seeing". Examples probably will help you understand.
今見える means you can see something at this very moment, and implies your view was not obscured. If I asked you by phone where you are, you would say something like 「今、道の向かいにコーヒーショップが見える」 (Right now, I can see a coffee shop across the street).
見えてきた means that you can see something that you weren't able to see before for some reason. Perhaps you couldn't find that coffee shop due to thick fog, but it's finally starting clear 「霧が晴れてきて、やっとコーヒーショップが見えてきた」(The fog is starting to clear, and I can finally (start to) see the coffee shop.). Or if you were driving to the baseball stadium, you could tell your passengers 「ほら、スタジアムが見えてきたよ」 (Look, we can see the stadium.) as the stadium finally becomes within view.
Note that you can also use this for non-physical "seeing". Say your at work trying to tackle a difficult problem, say trying to run a project with a limited budget. After days of struggling, pieces of the puzzle fall into place, you feel you're on track to solving this problem. 「正解が見えてきた！」(lit: I'm starting to see the answer!) you might exclaim.