"Would it be OK if I asked you out sometime?"...Why ask this?

Anonymous
Why do people, before asking someone out, sometimes ask "Would it be OK if I asked you out sometime?"? I just watched the very first episode of Friends where Ross says that to Rachel. I mean, isn't that a mood killer? Why do people ask that to someone they wanna ask out because then won't they be expecting you to ask them out in the future? Not to mention the fact that they already know you now like them and may even wanna play a few games with you.
"Would it be OK if I asked you out sometime?"...Why ask this?
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