Since the MacBook Pro with Touch Bar was announced a couple weeks ago, there's been plenty of speculation on why exactly it exists and what role it will serve in the Mac lineup of the future. The same kind of questions arose when the Force Touch trackpad was added to the MacBook Pro a couple years ago. There, the answer to "why this?" seemed fairly simple: as a non-moving part it eliminated internal space and made better use of its ever-increasing surface area, since an equal amount of force registers a click on any part of the trackpad.
The answer for the Touch Bar isn't so obviously pragmatic. It doesn't save space, as it occupies the exact same area as its predecessor function keys; the low end model that retains those keys reflects that exactly. And since the machines still aren't even in the hands of reviewers, the jury is still out as to whether the touch interface features will make the Mac more of a joy to use. Nobody knows if the Touch Bar will be liberated from the MacBook enclosure and put in standalone input devices, like when the Force Touch trackpad became the Magic Trackpad 2. (And unlike the Magic Trackpad, given the components involved, it seems nearly certain that a standalone Magic Keyboard 2 with Touch Bar will not be a $0 build-to-order option with the iMac of the future.)
Nevertheless, I think that Touch Bar technology will spread to all Macs, even if it comes at a cost to consumers. The reason is the problem that I believe served as the entire genesis of the Touch Bar technology: someone inside Apple looked at the glyphs on their function row keys and said "why do we keep letting these become obsolete?"
There was once a time when that wasn't a concern; Apple's desktop and laptop keyboards just had plain, numbered function keys. This was even the case when Apple started assigning essential features to them, like when Exposé was introduced in Panther. Its features were originally mapped to the blank F9, F10, and F11 keys, and I learned to use them by feel on my 2003 PowerBook G4. (It was super easy, as the function keys were physically grouped into F1–4, F5–8, and F9–11.) But the next wave of Apple hardware brought dedicated media playback keys in the function row, and they landed on the only glyphless function keys available: F9–11.
Obviously my keys didn't change overnight, but their associated actions did. Software "broke" perfectly good hardware. And other mismatches constantly arose. Hardware keyboards could easily outlive the features printed on them. Until just last year I was still using a keyboard that had a Dashboard key on it (yes, Dashboard is still technically alive, but just barely). And uniformity is impossible, as today's laptop keyboards with backlight controls put them on F5 and F6, while Apple's standalone keyboards — designed to work with any Mac — just offer blank space there. "This keyboard doesn't have backlighting, so, I dunno, have a couple extra buttons." It's inefficient design.
So rather than scrap the function keys as hopelessly broken, Apple is trying to turn a weakness into a strength. The function keys keep changing every couple years, and it's a problem. Instead, make them change every couple seconds and it's a delightful solution. Maybe someday the Touch Bar will vanish into a fully interactive keyboard. After all, the original Mac keyboard had no function row and neither does the iPad Smart Keyboard. iOS goes as far as dropping number keys, hiding them in a separate mode. I can't predict the future of Apple's input devices beyond the Touch Bar, but it seems a safe bet that they'll be capable of changing — quickly.