What are people trying to imply by using the phrase "just saying"? It sometimes seems they are trying to lessen the negative impact of a prior statement, or perhaps adding legitimacy to it. Perhaps it is only used because it is popular and makes a person sound contemporary rather than cerebral.
I've most often taken "just sayin'" to mean "I hold the stated belief, but I don't have an imperative to act on that belief."
Similarly, it may indicate a desire to hold and express the belief, but not to want to argue for or defend that belief. This could be because the belief is phenomenologically obvious [She was hot. Just sayin'.], because the belief may be based in faith [I need Jesus in my life. Just sayin'.], or because the argument may be too complex or boring to have [There's no general algebraic solution to the quintic polynomial. Just sayin'.].
So depending on the context, "just sayin'" may indicate that the speaker sees the belief as possibly wrong [I'm just saying, I don't think the party will be fun.] or as so obvious its legitimacy can't be questioned [I'm just saying, that dress is clearly white and gold.].
As OP notes, "Just sayin'" is familiar today because it is popular and makes a person sound contemporary.
A more formally-minded speaker (or one from a bygone age) might use, for example "I merely make an observation" (here are a few dozen who did exactly that).
The exact meaning (and degree of emphasis added/subtracted) will vary according to context. Sometimes it's almost "aggressive" (the point I'm making obviously conflicts with something associated with you). Other times it's more "placatory" (I really don't want to argue with you, but I just couldn't stop myself from saying what I just did). Sometimes it's not much more than a conversational filler (akin to "um, yeah, well...") or a conjunction (nevertheless, still, but).
“just sayin’” is a vernacular formulation that's reached the status of "meme", and it's probably so popular because it serves a variety of purposes well. I hear it expressed often in casual debate and in such circumstances will use it myself. “just sayin’” is used, not to “lessen the negative impact of a prior statement” but, rather, to lessen the negative impact of that prior statement’s reiteration. Although it can be employed with an aggressive tone its actual purpose is to downplay the contentiousness of the speaker’s insistence that their prior statement is the "last word" on the subject, so then "just sayin'" can be a soft way of having the last word in a casual.
Here are some additional takes on the meme:
From, The Urban Dictionary.com just saying
First Definition: a phrase used to indicate that we refuse to defend a claim we've made---in other words, that we refuse to offer reasons that what we've said is true.
• A: "You shouldn't hang out with her anymore."
• B: "After all she's done to help me? Wouldn't that be pretty mean and selfish of me?"
• A: "I'm just saying, you should quit hanging out with her."
Second Definition: When saying "Just saying" anywhere in a sentence, the beginning, end or the middle, it justifies your sentence as redundant, but in your mind you think its concise.
From, The Examiner.com I’m just saying
• The person wants to share information without having to explain it further.
• It's really an attempt to make an extremely weak argument to make the speaker's point good in the face those who might argue against it.
• The person using this term doesn't have to or want to invest any amount of critical thinking skills and logic to support the position or to offer an explanation.
From, painintheenglish.com I’m just saying
I am from Northern Maine (Aroostook County) and that is a favorite statement here. I think here it has four meanings.
1. The statement is in my opinion.
2. My opinion is all that matters.
3. Pointing things out that are mostly obvious to another person.
4."getting the last/final word in".
• In almost all instances that I have seen, this phrase follows a comment which could be critical, controversial, incendiary, or rude. Tacking it on the end of a sentence usually implies, "but don't get defensive," or "I don't want an argument." However, because it withdraws the critic from the critical statement issued, this makes it all the more frustrating to reply to.
Additionally, and so far completely ignored, “I’m just saying” can be delivered humorously, and with great effect,
• Basically "I'm just saying" or the more colloquial, I'm just sayin', is a humorous way of pointing out a flaw in logic, often delivered as a quip or a jab. The intended purpose can be to stop someone from continuing an already illogical argument or statement, or as a way of rebutting the statement once it is completed. When properly delivered, it is a devastatingly witty means of ending an argument that also makes the intended mark rethink their train of thought and provides fodder for any bystanders.
I have seen this in a sarcastic sense where the person is defending the original statement as true or self evident but it is understood the other person is so convinced of their views they will not accept it. For example (in a political context):
The board reviewing this development proposal did say that it would increase traffic congestion in the area