If you look carefully, you'll notice that the usage messages of package functions are nicely formatted.

Notice the nice italicised and subscripted $x_1$. If you actually look in the package, you'll find a usage message that doesn't have any formatting at all, and even differs from the version ?DelaunayTriangulation
gives us.
If[Not@ValueQ[DelaunayTriangulation::usage],DelaunayTriangulation::usage =
"DelaunayTriangulation[{{x1,y1},{x2,y2},...,{xn,yn}}] yields the (planar) \
Delaunay triangulation of the points. The triangulation is represented as a \
vertex adjacency list, one entry for each unique point in the original \
coordinate list indicating the adjacent vertices in counterclockwise order."];
So where did the formatting come from? It turns out that Mathematica includes an alternate set of usage messages defined elsewhere. These are nicely formatted. The Not@ValueQ[...]
is there to prevent the nicely formatted messages from being overwritten by the package.
The file that contains the formatted messages is located at
$InstallationDirectory/SystemFiles/Kernel/TextResources/English/Usage.m
Use
FileNameJoin[{$InstallationDirectory,
"SystemFiles", "Kernel", "TextResources", "English", "Usage.m"}]
to get an absolute path from Mathematica.
Regrettably, it seems some errors have slipped in and sometimes the formatted messages contain less information than the original ones... just like in the example above.