Broadly speaking, if you have a usual free standing washing machine there is room for a plug and socket.
But if you have the fancy built-in, integrated type, then you will need the flex outlet.
If you have a new washing machine delivered which includes fitting, then they will expect a socket, and will almost certainly refuse to cut the plug off and wire it into the flex outlet. They would probably allow you or an electrician to do it though.
The same would apply for the removal of the machine if it needs repairing or replacement.