I guess there are different reasons why users should or want to provide their email address on their own. In the moment only admins can enter email adresses.
An option to allow users to enter their own email adress would be helpfull.
I think LDAP is a centralized directory mostly used in companies to hold all kind of user data. Usually well defined update procedures exist to care of that data, so that it would be counterproductive in most cases to allow Nextcloud users to directly edit data from my point of view.
Nevertheless if an administrator wants to allow LDAP edits the LDAP write support app can be used for it. (Iām personally not using the app nor have I tried it, but the app name intends to be what you are looking for 
)