Hi,
Scan chain is disconnected before any change in placement so that the order of scan flip-flops will be optimal.
It is not just because it is done before CTS - it is because of he placement changes to be done (here, it is because placement optimization i.e. no placement optimization after CTS, then, no need for scan-chain to be deleted & re-connected).
Suppose you don't disconnect before placement changes. Once you do changes to the cell placement, you might have 1 scan FF at say, the far right of your design core while the next connection is to another scan FF at the left edge of your design core. And in between them are lots of other scan FFs which could have been used.
Now, routing like this can drive you nuts, right?
Simpler would be to route scan connections from the far right, then to the scan FFs in the middle, and later on to the scan FF on the left edge.
My 2 cents worth.
Best regards.