The design-time values in an SSIS package can be overridden by package configuration, so be sure you check all the package configuration settings and understand which order they are applied in if there are conflicts.  And all of these configurations could be overridden by command-line parameter settings in the SQL Agent job step (or whatever else invokes the package).

One use I can think of for the connection string expression is that if the connection manager is configured to point anywhere else when the package is loaded, the expression will override that configuration at runtime to set it back to the design value.

The self-referencing variable expression makes no sense, although you can configure the variable expression property so it is something else at runtime.

The only reason for either of these methods that I can think of is to confuse someone trying to reverse-engineer the package.  The much more likely explanation is that the package authors were confused to begin with.