Using the wall to keep GDR citizens in was definitely a big reason. But the context is more complicated. The GDR spend a ton of money on free education that was better than what could be found in the west. People that had some „low quality“ jobs before had the opportunity to become highly skilled members of society, all paid for by the state. That was a huge investment, but worth it for society. However the west targeted this specifically, giving people that went from the GDR to the west huge amounts of free cash.