Why Putin annexed 15% of Ukrainian territory?

On Friday, Russian President Vladimir Putin signed decrees declaring four Ukrainian areas to be part of Russia

a move that has been widely criticised as a "illegal annexation."

Putin made the announcement in the same chandeliered St. George's Hall of the Grand Kremlin Palace 

where he declared in March 2014 that the Ukrainian peninsula of Crimea was part of Russia

Russia has decided to acquire the lands, which account for approximately 15% of Ukraine. 

Many believe that this will alter the final outcome of the seven-month conflict.

Ukraine has reclaimed significant swaths of land and prompted a Russian retreat.