Not every website, but certainly a great number of them today, are being designed or redesigned to be reactive to the user's devices of choice. Some call it responsive design, in that the site responds to the form factor of the platform on which it is being viewed. Thus, a site 'works' fine on a large PC or Mac monitor, on an iPad or other tablet, or on an Android or iPhone, for example.
We care not whether you call this responsive design, reactive site development, or re- anything else you like. Key is designing and building your site to meet the needs of your audiences, whoever they may be. Knowing the audience becomes more important than ever, including the device preferences they exhibit for viewing your site, not to mention those much more important factors, desired content and functionality.
Do the homework - look at your traffic and gain insight into device preferences. Then decide on the level of investment needed to make your site better respond to core audiences needs. The site may not look exactly the same on each device, but it will provide a consistent level of satisfied user experience, which is what we are all after in the first place.