First, if the air is too dry, it will try to pull moisture from your water-based ink, which can, in turn, cause clogging issues.
Second, the proper humidity keeps your print head from overheating. Some operators assume if they are not having clogging issues, their humidity level is fine. But, if humidity is below 40%, your print head will tend to overheat. A print head will cool itself by doing repeated automatic head cleanings to force water (ink) through the head.
So, less than proper humidity will cost you in downtime and wasted ink as well.