Usually, when someone say 来到 someplace， the speaker is stay in the same place. e.g. when you say，他来到了上海。 you may be in shanghai when you speaking. But when you say 到，you may mean 去到or 来到，so you may be not in shanghai now. Hope it helps.

来到 its connotation means you were/are already here. I don't think it refer to some grammar.

Let me tried to answer your question. " 来"，this word is not only verb. Its basic meaning is ""come or arrive. But It depends on the context. E.g: A: Hi I would like to visit your place tomorrow, is it convenient for you. B: 来吧（come as you like). E.g: A: who can answer this question? B:我来。（ let me do it/let me answer this question）... hope it helps.