The demos at Michigan Central is to make way for green space and pathways to connect MCS, the Book Depository and the parking garage on Bagley.
There's renderings here.
As mentioned on the previous page, the demo at Michigan and Washington was likely demolished for a rumored apartment building that's been briefly mentioned in the media the past few years. The parking lot directly north was recently acquired by the owner of the demo'd building and has been mentioned to be part of the plans. There hasn't been a public announcement yet so that's really all we know at this point, but I'd assume we'll hear something in the next few months.