First you never forget to shut them off, saving energy and reducing dust build up and risk of burning up your fan motor or worse (bathroom fans need periodic cleaning out of dust just like dryer ducts do).
Second, you will use the fan more often because you may want to leave the house but then don't turn on the fan because you don't want it running the whole time you are gone. This is especially nice if you take showers before work because you can run the fan for whatever time is needed to get all the moisture out even if you have to leave the house before it is done. This helps tons to prevent mold and mildew in your bathroom. They are helpful if you take showers before bed too!
The new electronic switches are not super expensive (less than $30), and have a range of times from which to choose. #home