AFAIK those are just comments. Anything between <!-- and --> serves only as a note to those who have to work with the html code (the designer/programmer), or as 'private' code. Ie, an internal search engine to that domain or company might honour the tags.
Most major spiders will ignore it if not all. I remember comment tags used to help with Infoseek a few years back, I don't think they have any use anymore in seo nowadays.