In the past, emails were used primarily for correspondence. Just take a look at your personal email account — sort by date — oldest first. You probably still keep the messages from your old friends, and memories of the good old days.
Since the email address itself serve as a unique identifier, it is also commonly used/ requested when we register accounts for different kinds of online services. After registration, the service provider can, through the same email address, send us updates about the account or subscribed service, and sometimes advertisements as well. If we forget the login password (which happen from time to time), the same email address is instrumental in the process of resetting the password.
Some email addresses can serve additional purposes, i.e. verifying employment status and place of work, as well as proving professional credential. Those are work email addresses which end with the company or organization domain, e.g. firstname.lastname@example.org. Some service providers require the new users to provide a valid work email address. Such requirement sets a threshold to exclude people without corporate affiliation from being a user.
Note: there are a number of online services like SaaS which cater to a company’s HR, finance, IT, sales and marketing needs. It is justified for the company team to provide company email address to the external service provider, since it is essentially a “corporate action”. On the other hand, some online services mainly cater to individual users, but yet request the provision of work email address for the purpose of verifying employment.
Below are some examples (services catering to individual instead of corporate users) in which work email addresses are requested:
Blind is an anonymous community app for the workplace. Every user has to provide work email address in order to validate employment status.
Fat Llama is an online rental marketplace. Borrowers can boost trustworthy score by providing work email address.
Cozy is a property rental software. Prospective tenants can have employment status verified by work email address.