On the Internet ideas are quickly spread and adopted. Somebody comes up with something new and then others follow, and whatever that "something new" was becomes part of conventional wisdom, rarely to be challenged again.
For instance, why would you nowadays write something like "Welcome to my online store" or "Welcome to the customer service department"? To write "Welcome to…" seems so outdated and is just irrelevant to site visitors. And of course it might be taking the place of another much more relevant keyword. I chalk this down to conventional wisdom, i.e. that this sort of writing goes unchallenged.
"Welcome to…" also implies the notion of "site" vs "page". It’s about pages, folks, not sites. The major search engines index pages not sites. Many visitors may only visit one page on your site, and leave if they don’t like what they see; they don’t care about your site, just whether this one page answers their questions or not.
Google has about 32,900 Yahoo! Store pages indexed with "Welcome to" in it.