Companies in the same field that have links pointing to each other's Web sites increase their rankings in the major search engines such as Google, Yahoo, Ask, Alta-Vista, MSN, Teoma, DMOZ... etc. Being in the same field such as travel, and hotels increases search engine ranking more than if two web sites are in different, unrelated fields of expertise.

Google explains this clearly on their webmaster-help pages at: http://www.google.com/webmasters/4.html#A1