It's because sometimes, such as when Apple takes a year to update a line, that you can find an ultrabook that costs half of what Apple is offering for similar specs (or in some cases, same price, much worse specs).
It's not as severe as it used to be now that returns are diminishing on new generations of components, but even three/four years ago, it could be startling.
Take a look at the MacRumors buyers guide release date graph for the pro-line https://buyersguide.macrumors.com/#Mac
It's not as severe as it used to be now that returns are diminishing on new generations of components, but even three/four years ago, it could be startling.