Overgrown nails are easily caught and could result in loss of a toe, or even bleeding to death. A bird can bleed to death in a matter of minutes. Only an experienced bird groomer should groom your bird. Hookbills can become overgorwn to the point of interfering with eating. They are filed quickly and painlessly.
Bathing is part of grooming also.
If no source of bathing water is supplied, you may find your birds splashing happily in the drinking water. This is unsanitary and should be avoided by making a dish of water available a few times a week. Some enjoy misting in addition to, or as opposed to bathing. Lovebirds love to bathe and this activity is worth watching and observing. A seed-only diet most assuredly will result in disease, and more likely, a shortened life span.