There is a mantra that you will often hear, in a few different forms, that goes Write Situations, Not Plots.  I've repeated it myself a great many times.  It is excellent default advice; it describes the most useful ground state for preparation.

It's also not actually true.

You will write scripts (plots), situations, and sandbox elements.  That's just a thing that's going to happen.  Let me define those for you:

• A script is a segment of the fiction of play in which, barring major derail, the ending is known in advance, and wherein the role of play is to embellish, roleplay out, complicate, and contextualize the fiction.   You're being sent on a mission to kill Undead Hitler, and unless you die on the way, you're going to have a showdown in that bunker where der Furher Draugr will eat a (giant fiery magical) bullet; got the boss battle map right here.  We all know where this is going.
• A situation is a segment of play wherein the ending is not known in advance, but there's some degree of impetus to create one.  In it's simplest form, this is "Someone comes to you with a problem; there's a hook that involves you, it's going to get worse unless you do something, the particular something you should do is not predetermined". 
• A sandbox area is a chunk of fictional stuff wherein there's no particular pressure to resolve anything, but there is fun material to engage.  The characters are wandering the high hills in search of treasure, and there's some weird-ass ruins up here where purple lightning bounces around between big black metal poles.   There's a town with lots of shops, and the GM has lists of everything you can buy and the shopkeepers; wanna go shopping?

All of these can be nested within one another.   Let's use a video game example:

In Skyrim, the world as a whole is a sandbox.  It's an open-world game, after all.   And it is chock full of scripted quests, from the main story to each individual "I need something!".  It doesn't really have any situations per se, just scripts with multiple endings (including the derail of "I murder the whole damn village"), because...  Well, because it's a video game.  

If Skyrim were a tabletop campaign, but structured to feel the same, it would still be a sandbox at the setting level.  And a campaign there would still have a giant overarching script, which would also likely be "You're going to rise to great power and kick the hell out of Villain X" (this is, by the way, why a d20 level-based game is just fine for this; "rise to power and beat the shit out of the villain" is the precise territory that d20 level-based games live in and dominate the shit out of).  But the weird tangles of bullshit going on in the various towns would almost certainly be translated better into situations - you'd be able to make whatever judgement calls you wanted, and resolve that stuff all sorts of different ways.   The game would become a thingie wrapped in a whatsit parked on top of a whatchamacallit; different stuff at different levels.

So why do situations get stressed so hard by advice-givers, huh, smart guy?

Why, I'm glad I asked myself that in a funny voice!  And the answer is because there's a long tradition of doing everything as scripts, and doing it really fucking badly.  And by really fucking badly, I mean "The GM has right answers and solutions in their head, keeps those concealed from the players, and sometimes in fact conceals that the setup is a script in the first goddamn place, leading to everyone beating their fucking heads against the wall until they quit gaming with that utter asshole, who in turn may be upset because isn't this what they're supposed to be doing?", really fucking badly. 

To implement a script, you make sure the players are onboard with there being a script around, and you show that script to them.  You have the King give the characters clear marching orders.  You tell them exactly what the prophecy says.   There is no such thing as "too obvious" here.  And, in campaign play, you also do it very much like our big Nordic example video game does - you put the big one at the "top end" of play, and let the characters run around jumping into sandboxes and situations all along the way.

Cool?  Okay, back about your business.

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Oh, wait, no, hang on:  I should plug my relevant thing here.   Here's Situations For Tabletop Roleplay, which gives you a bunch of fill-in-the-blanks templates for interesting situations (and, actually, some that verge on being scriptwriting tools!  Gasp.  Shock.  Horror.  Anyway.)