share buyback.

Can I please first say that I have researched why companies but back shares but I appreciate someone with more indepth knowledge to explain why a company would lose its cash reserves by buying something it already owns. Why would it not invest elsewhere or expand operation, e.g. by way of acquisition? To me it looks like investors giving a company money to undertake activities to increase profit & value, but the company rejecting it by returning the money. This is contradiction in turns; the company listing itself on to go public & then pushing public away!