My personal motivation for purchasing a new computer is usually related to the screen. A higher resolution and larger screen is attractive to me. 27 inches seems about right, but maybe I could go larger too. I work at a computer all day, so this type of thing really matters to me.
Gamers have many different reasons for getting a new computer and probably upgrade more than any other computer users. Engineers often have higher processing needs, but still tend to use a computer for a number of years before upgrading.
Casual users have many fewer reasons. My advice for casual users is that if your computer is old enough that the latest operating system upgrades won't run on it, it is definitely time to upgrade. But upgrading to a new entry level machine is a fine upgrade.