Who needs a WebID?
A person needs a WebID so they can log in at applications and share data with other parties such as other people, organisations and applications. An example of a person's WebID is use.id/john.
Organisations need a WebID if they want to let others (e.g. customers, employees, suppliers, ...) share information with them. An example of an organisation's WebID is webid.digita.ai
Apps also need a WebID if they want access to the data of others*. Apps are a bit special because they also need to define what data they require in order to function properly in a machine readable format. This way, when a user logs in, the user's authorisation agent knows what data it should share with the app. An example of an application's WebID is webid.app.use.id.
What is a WebID?
Portable data vaults and authentication without a vendor lock-in
Your own (virtual) data vault...
You can consider your WebID as your own virtual data vault. You can connect multiple (Solid compliant) data vaults (a.k.a. pods) and OpenID Connect identity providers with your WebID. A WebID allows you to have a unified view over federated data.
... shareable with whatever party of your choice
When you want, you can easily share selected pieces of information from your WebID by sending third parties a link to those pieces instead of sending them the actual information. This way, companies or friends have a direct connection with you or the single version of your truth. This means that to keep in touch with your friends or the companies you like, you don’t have to rely on platforms such as Facebook or Instagram. Another benefit would be that, when you want to connect to a company, you don’t have to re-enter the same data over and over again or worry whether they will keep it up-to-date (e.g. when you move house).
... stored wherever you want
Of course, changing your WebID (e.g. from use.id/john to my.id/john) will bring the same kind of trouble that you get when changing your email address or phone number. However, because the actual storage locations (i.e. pods) are abstracted, you can easily move your data from one storage location to another without any concerns.
... browsable by whatever app you want
You can access your WebID, like the data that is associated* with it using any Solid compatible app just like you can use apps like Outlook or Thunderbird to access your emails. Certain apps will give a specific view on specific kinds of data whilst others might even give you an overview over all your data.
* This could be data of yourself or data of others that is shared with your WebID.
... putting you in control over your data
A WebID also contains a registry of who can access what. By doing so, it effectively allows you to store and control your privacy preferences like you would manage the permissions of your apps on your smartphone.