This difference is very important.
The password of a gallery, set and activated in the gallery settings, is there to protect your gallery from possible unwanted visitors. If it is activated, your client must enter it when calling up the direct link to the respective gallery. Only then are the pictures of this one gallery are visible.
With a client login, however, your client can use a password to access an overview of all galleries that have been activated for them. You can create such login codes in your account settings. There you can also change the client login at any time or add or remove galleries to a client login.
The advantage: your client does not need the latest, direct link to a new gallery every time, but logs in with his client login and automatically sees all new images available for him. It is planned that your client will be notified automatically when new galleries are available for him, but it is currently still necessary that you inform him directly.