Why do players think after they have made a plan? I have seen many live competitions from top GMs, and I can usually see one thinking 15 minutes straight, then doing a tactical move. After some thinking from the other player, he does the main move, the one the engines say it is the best, and probably was considered by the first GM. I keep seeing GMs thinking, spending more time, even if the opponent plays what they wanted. Double checking might be it, but still, isn't it valuable time spent on checking an already checked position?
I had the same thought myself. For a couple tournaments, I started playing lines which I had calculated out with about 10 seconds of thought on each move. My results were terrible.

The reason, I think, is that the extra move being played on the board gives you quite a bit of added calculation power. I now take more time on the moves following a long think and I frequently encounter errors in my previous analysis, which I either have to find fixes for or find a new move.