Why does the anode solution contain Sn2+ in a Sn-Cu voltaic cell?
A voltaic circuit needs to be a complete circuit first an foremost. So, to answer your 2 main concerns, think of a voltaic cell as a electric circuit:


  In one half cell, we have a $\ce{Sn}$ anode in water connected to a wire.


Water is not a good conductor of electricity, dissolved ions are needed, such as the $\ce{Sn^{2+}}$ ions are needed to facilitate the movement of the electric current in the first place. Also, the presence of the ions allows the overall redox reaction be spontaneous to provide the electric current (ChemWiki).


  The wire is submersed in the other half cell which is simply $\ce{Cu^{2+}}$ solution


According to the ChemWiki Voltaic Cells, the metals plates provide a surface for the reactions to occur on, a wire just submersed into the solution provides a very small surface area for the reaction to occur.

A diagram from the UC Davis ChemWiki page Electrochemical Cells helps explain this (the example uses zinc instead of tin, but the principle is the same):