Basically what the title says. I've been experimenting with the Companions on the PTS, and one of the things I noticed is that while we can give them a customized outfit using the Outfitting system, there is no option to customize their helm.
While I do understand that these Companions have been created and written with their own story and narrative, and placing a helmet, hat, helm, or hood on them might hide their face and disguise their appearance, which could be contrary to their character as written, many players will be going through these questlines more than one time. Myself included. In addition, some people are excited about the Companions for more than merely their potential as combat assistants or their stories as written by the ESO staff. Many people in the housing community have been looking forward to being able to place them in housing, and have been overjoyed at the ability to customize their appearances. For them, being able to hide their identity underneath a helmet makes them much, much more versatile, and much more appealing.
For myself, I have a lot of characters, and some of them I really look forward to acquainting with the new Companions and their stories. But for others, I have been hoping to use the Companions as stand-ins for personal retainers, armspersons, accomplices in their nefarious activities, apprentices, and other such things as they go about their adventures in the realms of the Elder Scrolls. Being able to hide their face with a helm is really a key part of this, because it lets the Companion be a different "person" for each of the characters. I can't rename them or give them nicknames, but just being able to assign a helm to that outfit slot instead of being locked out of it would be huge in expanding the versatility of these new Companions for the player base.