It can be useful to switch between different sets of settings in
mu4e; a typical example is the case where you have different e-mail
accounts for private and work email, each with their own values for
folders, e-mail addresses, mailservers and so on.
The mu4e-context system is a mu4e-specific mechanism to allow
for that; users can define different contexts corresponding with
groups of setting and either manually switch between them, or let
mu4e determine the right context based on some user-provided
function.
Note that there are a number of existing ways to switch accounts in
mu4e, for example using the method described in the Tips and Tricks section of this manual. Those still work — but the new mechanism
has the benefit of being a core part of mu4e, thus allowing for
deeper integration.