Dry/wet determines how much effect you can hear. Dry means no effect is heard… Wet is full effect.
The dry/wet know is best to remain variable as some effects sound better dryer and some wetter, eg a full wet flanger sounds very noobish but can add to a track if you use a little more reserve. Conversely if you are using a beatmasher or roll effect it sounds better full wet so you get the stutter effect.