The most important reason for using texi2latex is customisation. For people knowing LaTeX it is almost trivial to implement their own layout. Changing the default font face, for example, is a one-liner. The same is true for page margins and headers. Since texi2latex's output is plain LaTeX with only a few quirks, you can provide your own preamble to adjust everything to your needs. See Customisation, for more information.
Another advantage of texi2latex in comparison to makeinfo's TeX output format is that some things are difficult to implement in Plain TeX but are already available in LaTeX. Floats, for example, are not yet implemented in Texinfo, although the specification includes them. Their realisation with LaTeX is almost trivial. Other examples include full PDF support, full support of non-English languages, and decent line-breaking of URLs.