What the hell does that mean?
I usually hear it from someone who sees themselves as in a position of authority.
Security guards, clerks, aerobics instructors, receptionists etc. Cops.
Me: “How do I get back to I80?”
Gas station clerk: “I’m gonna need you to go down to the third light…”
Like that.
Or, (real, on tv)
Guys in a car at the NASA security gate: “We want NASA to help us see Santa from space.”
Gate guard: “I’m gonna need you to go to building B…”
Peace,
mangeorge
“Hey Peter, whaaaats happening? Yeah, I’m gonna need you to go ahead and get those TPS reports in with the proper cover sheet”.
Sorry.
Damn! Beat me to it!
I can’t help but hear Lumbergh’s voice when I read that phrase, but I have to admit I’ve never heard it used the way the the OP presents it. In the examples he gives, I’d expect to hear “you’re gonna need to…” as opposed to “I’m gonna need you to…”
I used to preface my statements with “you’re gonna wanna…” “take a left” or “drop that bag full of badgers” or “not walk into Mordor” or whatever. I have no idea why I said that. It was just some weird affectation I picked up along the way. Then one day a friend turned to me
And said “Really? Are you sure I’m going to want that? You don’t know me!” she was joking of course, but pointing out that it was a fairly ridiculous thing to say.
So I guess my answer isn’t so much helpful but there ya go. You’re gonna wanna read the rest of the thread for a better answer.
Also, much later I had a habit of walking around the office with my ridiculously large coffee mug handing out orders. They started calling me Lumbergh.
I hear phrasing like that from aviation guys… I kind of associate it with ATC instructions…“I’m going to need you to taxi on Charlie and hold short of 24-left” and stuff like that. In ATC, the controller really does need the pilots to do what they ask, because of how they are choreographing all of the aircraft under their control in order to dispatch and land flights on time, or properly deal with a developing emergency.
Exactly!
That’s the opposite in Chefguy’s example.
I had to wiki ol’ Bill Lumbergh. I feel so left out. 
In linguistics this kind of phrasing is known as a “discourse modifier,” specifically, in this case, a “pragmatic softener.”
However, “I’m going to need you to” and “you’re going to want to” are pragmatically different. The first, as the OP says, is used by people when in some kind of position of authority when giving commands. It mitigates the expression of authority, and that can be for various reasons. A security guard on private property is a great example, because he or she can’t (technically) arrest you. However, the police will also use it (sometimes even when pointing their weapons at someone), as a token gesture of respect. (And probably as an attempt to help de-escalate a situation.):
“I('m gonna) need you to step away from the vehicle,” etc.
“You’re going to want to” is not used for commands, so much as for matter- of-fact instructions. (Could it be that that is what the store clerk in the OP actually said?) He is not telling you that you may or may not go to the third light, but rather that you have no other choice if you want to arrive at your destination. However, because he’s (“just”) a store clerk, he embeds the proposition in a way that provides for your agency. (I.e., you go to third light not because the clerk is telling you to, but because you “want” (choose) to.)
One of the most common pragmatic softeners in English is “actually,” to express a proposition that is either contrary to or not necessarily aligned with the previous discourse:
“Actually, why don’t we just be friends.”