Allow use of secondary email address for the same activities as the primary email, e.g. for sign in, password reset, to approve a change of the primary email address.
I changed my primary email address and right after dropbox told me to reset my password, however the reset link could only be sent to the old primary email address which I no longer had access to. Luckily I was still logged in with another device, otherwise I would have lost access to my account with no possibility to restore it (according to FAQs).
If I would have been able to use the secondary address I could have sent the password reset link to this address or I could have signed into my account with the secondary address.