Just is a bitch mother that seeks to handwave away any potential problems.
If it’s just that, I’ll gladly step aside and let you do it. But if you then tell me you can’t do it, then you better sit down, shut the fuck up, and listen when I tell you something is more complicated than you think.
My best example of this is my father-in-law. He's an absolutely wonderful man, generous to a fault and all of that. Fancies himself a handyman. But I had to stop asking for his help with around-the-house projects out of sheer anger and frustration because every SINGLE time I made a decision about how to proceed with the job or pick which tool to use to cut something, he would stop and and ask, "Well, why don't you just do X instead?"
It didn't seem to matter if I explained my reasoning, or if doing X meant redoing a hour of work just to get back to where we currently were. I even attempted to humor him by trying things his way whether or not they were going to work out. (They often didn't). Eventually it ended in me saying "no" way more often than "yes" and he would infer that I was refusing his help and he'd throw his hands up and walk away.
Sounds like he spends a lot of time on StackOverflow.
I think it's not an uncommon opinion. And of course, Adam Savage is always great.
In some cases, it's diminishing or trivializing the work. "I just need to do X" trivializes the amount of work that X takes and in some cases implies that the person I am talking to is not competent / capable of doing it themselves (often implying that they should be).
Likewise, "they just did X" trivializes the work that the other person does.
Its a word I ̶j̶u̶s̶t̶ don't believe adds value to a sentence.