WIndshield washer fluid contains ethyl alcohol and some other chemicals that are specifically designed to be used for that application.
It won't freeze in the winter and won't evaporate in the summer. It lubricates the washer fluid pumps as they operate. IF they run dry you'll eventually destroy the plastic internal parts.
If you use water only, you'll also cause premature internal wear and your lack of a trip to the local wal-mart for a 99 cent bottle of washer fluid will cost you about $200 plus labor.
Buy the fluid. If you have to, you can dilute it about 50/50 and it will still be affective down to about 35°. But I would fill up full strength as winter sets in. If you live in a milder climate, you'll be fine diluting 50/50.