A washing machine in the flat (or the building) is most important if you want to rent out a furnsihed apartment and if you are not aiming for tenants who only wish to stay for a very short time (around a month, for example). Most tenants do not even consider furnished apartments without a washing machine.
A washing machine is even more important than a dishwasher. If you have enough space, installing both makes then definitely makes sense. Please make sure to get a washing machine which can be operated easily without reading complicated manuals and fitted with an AquaStop.
If you offer a furnished flat in a higher price range or if it is a flat that suits families, buying a dryer also makes sense.