If you want to change your email address, for example. For example, if you have been using the same email name since the early 2000s and now want to use your own name for more professional purposes. Or, let's say, it's like having two cell phones: one for work and another for personal use. Because both are gmail, you can change user / inbox directly in your Chrome browser.
It seems Gmail actually make Allow the creation of secondary accounts.
I'm not sure if this is still valid since the help file is dated 2015, but I do not think it's forbidden at all. What am I missing?