As others have said, it only puts the window to the side to give you a few seconds to finish something up and then join or withdraw.

While it may feel useless it does serve a purpose. It has happened to me a few times where I walk up to an NPC or Market Board and as my thumb moves to tap the confirm button the DF window pops and I hit Commence unintentionally. By putting the default position to Wait they fix this.

Same for the confirmation window when you withdraw. Several times I have been quickly throwing items up for sale and the DF window pops up as I'm moving the cursor over and hitting confirm and it Withdraws me on accident. These are both welcome changes for me.