Quite often, I see web sites that draw resources from multiple domains. Made up example:
Foo Bar Ltd has a main web site at foobar.com, which references files at fbltdcdn.com and foobar-rewards.net. They send emails directing people to foobarlatestmarketingslogan.com, which immediately makes me think "phishing scam".
Why is this a common practice, rather than using cdn.foobar.com, rewards.foobar.com, slogan.foobar.com, etc?
Static resources are often served from completely separate domains, when the website uses a naked domain (twitter.com for example). In this case, all the cookies set by twitter.com apply also to subdomains.
So, if the static resources were served from cdn.twitter.com, the cookies would also be sent to that domain. This increases the network traffic, and makes user experience worse.
The practical reason is to control sending of cookies. However, this can better handled by setting the correct domain (and path) on the cookies. This will greatly limit the cookie volumes.
One of the reasons I have seen is that someone decides to use thisIsAReallyGoodName.com for some reason. It may seem like a good (marketing?) reason at the time. However, this results in domain bloat within the organization. These domain may go AWOL as they may not get managed as a corporate asset. This can and does lead to link breakage.
As you have noted, it also leads to a loss of brand focus. In some cases it can damage the image of the company. I see this kind of mixed domain names in a lot of phishing email, so I, like you, will be skeptical of their communication. This is not good marketing.
I also see non-existent sub-domains being used in email addresses. This is unprofessional, and tarnishes the image of the company. (At least one large computer brand has this problem.)
For 3 reasons.
Fast browsing. Modern browsers download resources concurrently from each domain to have faster browsing experience. That's why some of the sites use foobar.com and cdn.foobar.com to split the site assets across few domains.
Cookies. Cookies are bad for caching. And they are not needed for serving assets (images, videos, css files etc.) under normal circumstances. So behind the scene, it is architected in such a way that foobar.com is served by application servers which generate cookies whereas cdn.foobar.com is served by light web servers which do not generate cookies.
Marketing purpose. Marketing folks don't care about how many domains you have. They will want to have a domain name most suitable for their campaign. In addition, a proxy domain can be created to track clicks before coming to actual domain.
