Shoney posted the "why" two posts above your post. Maybe you were still reading or in the process of writing your post & didn't get to read her "why" explanation?
shoney: "I agree. I asked my mom about it since it was around when she got married in the 60’s. Both parents are decendents of Irish immigrants and lives in nyc. They were not wealthy by any means She said it was a way to help the couple start out. The brides family would scrimp and save and instead of just handing the money over without a reception they would throw an elegant party. The friends and family would “cover the plate” so the bride and groom would essentially get that money back. Then the next person would do the same for another close friend or family member.
Granted weddings were cheaper My mom said hers was $25 pp and it was a multi course prime rib dinner.
She also said it was a way to socialize and go out for a fancy night. People didn’t have the money to eat out every week."
I grew up all around NY state. And I had never heard the term, "Cover your plate," until I read it on the DIS.

However, I am aware of the custom. Maybe it's that
term that is so objectionable to most people who aren't from here. It does sound very cold & financial - lacking "heart" which many have said is where their gifts come from. That is NOT the
intent of "Covering your plate."
Shoney's mom said it really well.

Here in NY, north NJ, & parts of CT, we do "big weddings" according to others from other parts of the DIS. That has always been customary at mostly all the weddings I've been to and have catered in all 3 states. (Except for a couple of young, still in grad school couples, who were naturally on a very limited budget.) Our "big weddings" consist of a hors d'oeuvres while waiting for the bride & groom to appear, open bar (negotiable,) a full 3 course meal or buffet (with choice of one of 2 entrees,) the wedding cake, usually a band for dancing, not a DJ, done at a wedding hall, a tent or huge backyard with hired waiters for the latter two.
While most couples aren't spending the amount an overboard Bridezilla might, they do spend quite a bit. They
want their family & friends to attend & celebrate with them. So YES, we do "big weddings" to include them and give them the full 9 yards.
In return, as most family & friends attending know how much the wedding costs probably run, they
gladly give some of the money
back if they have it, in the form of gifts or money. So the bride & groom don't go bankrupt having a huge wedding reception just to see everyone. Remember, in generations past, the bride & groom often didn't live together and already have much of the items being given. A nice 6-slice toaster oven or stand mixer would really be appreciated - because the couple wouldn't have been able to afford it since they paid for the "big wedding" to see everyone.
As Shoney's mom said, instead of being financially set back because of a "big wedding," it was a way to help the bride & groom back. It is an
honor & privileged to be invited to someone's wedding, knowing they could have cut you out. (General) You don't have to attend. Notice on the R.S.V.P.s there's no section to give a reason as to why you are NOT attending. REAL
family and
friends want help out in return
if they have the funds to do so.
I think some people are confusing or conjoining Bridezillas with the "cover your plate" concept, thinking a Bridezilla & groom would be eyeing each plate with a tit-for-tat attitude. Sure, there are some people like that. Yet, they are out there in
all parts of the country.

That has not been the case for the "big weddings" I go to. (Don't know enough about the ones I was at catering.) But, MY
friends don't do that. Do YOURS??? (General yours - that is.) They wouldn't be
my friends for long if they are like that. Especially if they know our mutual friend Susie recently lost her job due to downsizing, yet they STILL expected a gift to cover her plate. If they were that way, that's a
values problem, a superficiality, and having a lack of compassion & understanding for Susie, and I'd be seriously re-thinking about them as friends - because they'd do it to me too, when I'm down on my luck.
The parties & receptions I've been to where a Susie had lost her job, the bride & groom would fold her in a big hug, say how glad they were that she came, instead of staying home depressed. They'd hope she can forget about her troubles for the day, celebrating with them, get some more hugs from fellow friends, maybe get in some dancing and have a piece of the to-die-for wedding cake that can make anyone forget their troubles for a little bit.

If they seriously think the $14.99 Sterling silver gravy ladle that she had specially monogrammed with their initials as less than, because they spent $47 on her plate, sooooooo not friends of hers. And good riddance to them.
Susie deserves BETTER.
I'm tired of DISers bashing our metro-NY/NJ/CT "big weddings." I think many of you have watched the "big wedding" in
The Godfather, too many times, and that Corleone, Long Island mafia wedding image is stuck in your head as to what OUR big weddings are like - with someone keeping score. "We do a little for you. . . Later, you do a little for us in return." NOT!

[/QUOT
TL;DR all of it....
My questions weren't really answered other than it is a 'guideline' and very old (60s?) financing of a wedding. Anything else seemed an attempt at lack of common courtesy.
It is not clear of what cover your plate is vs putting a down payment on a house.
It is the expectation that is the big question.
And, yes, we have BIG weddings but pay for our own, do not expect others to gift to cover the costs.
We pay for our own and do not have a wedding we can't pay for up front.
Everyone lives differently; is this how it is done for every party etc.?
Funny, IMO.