The shift key isn't there just for making letters go "big" (uppercase is the right term). It's not there also for extending a selection (like clicking on more icons when cleaning desktop or even clicking the first and then the last item in a list to select all between). Or limiting angles to 45 degrees on Photoshop. Or to get a previous occurrence of a searched item (with cmd-shift-G).
The Shift key might get you out of troubles, quickly. Going back to history, seasoned users who used System 7 might even remember that they could bypass loading extensions during start.
So, some Mac users would know that Shift key can be held during start (just when the Apple logo is on the grey screen) to bypass third-party add-ons in the current Mac systems. But it's not just that; it also performs basic disk maintenance tasks, something which is otherwise achieved by booting into a recovery mode and running Disk Utilities. That's why, paradoxically, this Safe Boot takes a bit longer.
Additionally, it clears caches. It might be good if some of the apps or system parts aren't working properly with some cached content, and crashes or misbehaves. So these caches, which otherwise help the system and apps to load faster, are safely removed and then recreated with the next boot — and that's why the next one is also a bit longer than a usual boot.
The lesser use of the key is holding it when you're just logging in. As you also might have some start up items or extensions — they're called (quite Hollywood-y) Launch Daemons and Agents — these again are ignored when holding Shift.
And finally, when you're launching an app or opening a document, the Shift key might be used to alter the app's startup behaviour. Some apps would try to open previously opened documents; the Shift key will let you start with a clean plate. It is then often much faster way to troubleshoot an app which tries to open damaged documents and crashes during it (I'm looking at you, Word) — instead of usual, "nerdy" way of opening user Library (which is hidden by default) and finding the right preference file to trash.
So, remember that largest yet quite inconspicuous key on your keyboard and use it next time you're in trouble — it might save you a call to a geek friend or a trip to a repair shop.