Some company headquarters are open to public visits and even offer tours.
Some people go to company offices just to get a photo taken next to the company logo. It may seem silly, but people will go to Facebook's headquarters, take a picture of themselves there, and then post the picture on (guess where?) their Facebook page.
Much of Silicon Valley's fabled culture of innovation takes place within the confines of its suburban office parks, which are actually quite bland. Most Silicon Valley companies are too busy creating products and making money to offer much to tourists. Seeing the headquarters of so many legendary high-tech companies and venture capital firms in one place may be fun at first, especially for techno-geeks, but the novelty wears off after a few hours, and tourists should better stick to the campuses of Google (Mountain View), Apple (Cupertino) and possibly Facebook (Menlo Park in the Peninsula, a few miles north of downtown Palo Alto).
The only major Silicon Valley company that openly caters to tourists is Intel, which has a free small museum at its Santa Clara headquarters dedicated to its semiconductor business. At a lesser degree, Google has fun Android statues on-campus specifically aimed at visitors, and its colorful bikes and propeller hats are otherwise worth the trip. In addition, Apple operates a Visitor Center next to its brand new Apple Park headquarters—a colossal circular building which is by itself like no other—with an augmented reality model of the site and a balcony overlooking it.