When not empty all messages are written in a file with this name. When the file exists messages are appended. Writing to the file ends when Vim exits or when 'verbosefile' is made empty. Writes are buffered, thus may not show up for some time. Setting 'verbosefile' to a new value is like making it empty first. The difference with |:redir| is that verbose messages are not displayed when 'verbosefile' is set.