battery cycles of 15 is well within expected numbers. that just means it was tested a bit more than many new units are tested. if this is Apple Certified Refurbished, the same warranty as new still applies. you have nothing to worry about.
ANY device with a rechargeable battery needs to be charged before use.
That computer is a battery-CAPABLE device. It is not optimized as a battery-operated device. (It is NOT an iPhone.)
Your computer performs best when connected to AC power. It can use the full output of the Power Adapter AND when doing especially challenging work will also freely "borrow" power from the battery. In some cases, even with the power adapter connected, the charged state may decline during stressful work.
When used only on battery, your computer has no extra cushion of power, and may perform more slowly. However, for ordinary non-stressful tasks this may not be objectionable (possibly not even noticeable.)
Your MacBook Pro uses ‘smart charging’ to charge in the optimum way, and only when necessary. Plugged in is Not necessarily actively charging. There is substantial hardware and software cooperating on battery and charging issues. Connected power sources simply 'offer' their charging ability, and the computer decides what to accept and when. Simply asserting a charging voltage against one of the interface(s) will NOT successfully charge your MacBook Pro.
In general, you should ALWAYS connect AC power when it is possible to do so, and only run on batteries (which could be somewhat slower) when no AC sources are at hand. Your Mac will NEVER over-charge.