the disable sideways flight is for when the drone is in APAS or as its called bypass mode ,it is to prevent the pilot from over correcting sideways flight in bypass when flying between narrow obstacles , the drone will move sideways itself to bypass obstacles in its path ,but it does not have sensors on the sides of the drone to stop it hitting something along side it when it is stationary
you will have probably noticed that when it is in Bypass mode ,the control sticks need only a very gentle movement to make the drone go left or right ,and its very easy to give to much stick input ,and cause it to move to much ,hence having the disable sideways movement setting ,to prevent this happening