The aquisition is likely because they want to capatilize on the excellent Steam platform and use its technology to develop its own stable content distribution platform. They certainly have enough money to make the aquisition happen but I fear for Valve as they have never been tied to any large company before and it could be messy regarding retail distribution of their games.
Also who knows what google might do to Valve? If they owned them they could enforce certain things like deadlines for games e.t.c. which is not the valve way of working.