> Single logical site spread across multiple domains.
Is there really need for this? I get subdomains can help routing, but beyond that sites spreading over multiple domains are chaotic and phishing-prone. People get used to jump from foo.com to foo.net or scammyfoo.tk and enter their credentials if they look similar. I think that a big part of how password managers help is by keeping passwords from their users and not sharing them with any random domain that may read similar or misleading.
A common need for this is during an acquisition or merger.
It's fine and all to assume that domain is identity, but that doesn't actually map too well to relatively complex organizational hierarchies.
Ex - Bank A and Bank B merge. There is going to be a period where they have to navigate that two domains represent a single organization. It's often a fairly high level of effort to move to a completely new domain, and it won't be done overnight.
Yes - eventually you want to be back on a single domain, and I think there is definitely a world where this leads to some very bad patterns (HR and healthcare are two examples - if you've ever seen a login need to bounce between like 5 different domains because they've refused to actually do the technical work to consolidate back on a single domain, and treat the domain as marketing).
But it's a really valid spot to end up in, and is the most common cause of having a single entity spread out over multiple domains in my experience.
More common are multiple sites (which use their own domains for esthetic / brand reasons), but are actually hosted by the same SaaS provider and could therefore share authentication infrastructure.
Imagine an easy-to-use website builder for restaurants where each restaurant gets a memorable domain, and they let you order things online. It would be great for customers if they didn't have to enter their payment details and shipping address for each new restaurant they order from. Maybe they could even see opening hours and product availability for the closest restaurant to their address. There's no privacy risk here, as all these websites are actually on a single provider anyway. They're just multiple entries in some SQL database, each with a `domain` associated with them.
Is there really need for this? I get subdomains can help routing, but beyond that sites spreading over multiple domains are chaotic and phishing-prone. People get used to jump from foo.com to foo.net or scammyfoo.tk and enter their credentials if they look similar. I think that a big part of how password managers help is by keeping passwords from their users and not sharing them with any random domain that may read similar or misleading.