Bookmarks are "fragile" -- most people don't turn on the display of
bookmarks, and it's very easy to delete a bookmark while editing text. If
your userform code simply assumes the bookmark exists, it will throw an
error when it tries to insert text at a nonexistent bookmark ("The requested
member of the collection does not exist"). If you have to do a thorough job
of checking for bookmarks, and maybe recreating missing ones, you have quite
a lot of work to do.

When the userform code sets or modifies a document variable, the code
involved is much simpler. In the document, the DocVariable field can be seen
(assuming there's some default value of the variable when the document is
created from the template) so it's less likely to be deleted by mistake. If
someone does delete the field, the only consequence is that the variable's
value doesn't appear where it should.

Another advantage is that data can be retrieved (more reliably) from a
document variable than from a bookmark as the data may be inserted before
the bookmark and not actually be contained within the bookmark.

On the other hand, there are times when a bookmark can be used where a
document variable and DocVariable field does not work so well - inserting
items from a multiline textbox into the cell of a table is one.

One further advantage is that with a single docvariable (or docproperty for
that matter) you can have multiple DOCVARIABLE or DOCPROPERTY fields all
pointing to the same property, if you need the same data to be displayed in
different places (e.g. in odd and even page headers)