First off I found I have a subtle reaction to ALL dairy and only use plant based 'dairy' based products (which includes mayo). I never use any butter or margarine spread on the inside of a sandwich but I do use a mixture I make (1 part Earth Balance, 1 part coconut oil and 4 parts EV Olive Oil) on the outside when making a melted cheese (again dairy based). On the inside is the 'cheese' some mustard and some slices of pickle.

My dad used to instruct my mom to butter the insides of sandwich bread, in the mornings when she packed his lunch, as a moisture barrier, if the filling might make the bread soggy before lunch time. He may have learned this in the Navy.

I had a boyfriend who instructed me to toast his sandwich bread, when packing his lunch, to keep it from getting soggy before his lunch time at work.

I guess it's all just preference.

Not if they are a Moslem.

It varies these days depending on how I feel.
However there is one exception were the recipe has to be adhered to exactly
The perfect corned beef sandwich

Two thick home cut slices of bloomer bread (door steps as we call them) (never pre sliced bread)
Real slightly salted butter (Never margarine) spread thickly on bother inside faces (ideally this could be home made too).
Sliced red salad onions
Two thick slices of corned beef
Tomato sauce or ketchup, on the upper slice at least two tea spoons, or alternately serve the sandwich with a mug of Heinz Tomato soup for dunking
Build the sandwich, compress slightly and cut in to quarters
Enjoy.

Add salad to taste, and toast the outside of the bread if you like a crunch.

The King's Breakfast by A. A. Milne

The King's Breakfast
The King asked
The Queen, and
The Queen asked
The Dairymaid:
"Could we have some butter for
The Royal slice of bread?"
The Queen asked the Dairymaid,
The Dairymaid
Said, "Certainly,
I'll go and tell the cow
Now
Before she goes to bed."

The Dairymaid
She curtsied,
And went and told
The Alderney:
"Don't forget the butter for
The Royal slice of bread."
The Alderney
Said sleepily:
"You'd better tell
His Majesty
That many people nowadays
Like marmalade
Instead."

The Dairymaid
Said, "Fancy!"
And went to
Her Majesty.
She curtsied to the Queen, and
She turned a little red:
"Excuse me,
Your Majesty,
For taking of
The liberty,
But marmalade is tasty, if
It's very
Thickly
Spread."

The Queen said
"Oh!:
And went to
His Majesty:
"Talking of the butter for
The royal slice of bread,
Many people
Think that
Marmalade
Is nicer.
Would you like to try a little
Marmalade
Instead?"

The King said,
"Bother!"
And then he said,
"Oh, deary me!"
The King sobbed, "Oh, deary me!"
And went back to bed.
"Nobody,"
He whimpered,
"Could call me
A fussy man;
I only want
A little bit
Of butter for
My bread!"

The Queen said,
"There, there!"
And went to
The Dairymaid.
The Dairymaid
Said, "There, there!"
And went to the shed.
The cow said,
"There, there!
I didn't really
Mean it;
Here's milk for his porringer,
And butter for his bread."

The Queen took
The butter
And brought it to
His Majesty;
The King said,
"Butter, eh?"
And bounced out of bed.
"Nobody," he said,
As he kissed her
Tenderly,
"Nobody," he said,
As he slid down the banisters,
"Nobody,
My darling,
Could call me
A fussy man -
BUT
I do like a little bit of butter to my bread!"

I don't put butter on every sandwich. Not ham, not tuna salad, not beef, actually I don't think I put butter on any sandwich unless it's on the outside for grilled sandwiches.

My "uncle" who used to be a "chef" "invented" what he dubbed "The Cholesterol Sandwich". Two pieces of bread buttered on both sides turned into a PB&J, then dipped in batter and deep fried.
He was wanted for questioning in relation to dismembered body parts that were found. When the SWAT came to his house to collect him, he committed suicide by cop by running out of his house at them shooting a gun. I'm surprised my aunt and their kids didn't get hit when they blew his ass away.
That's a really weird 'buttered bread for sandwiches' story...

I’m a Brit….it’s not a proper sarnie without butter/marge!

Of course. Butter is an essential component of sliced bread.

I usually use low fat mayo., unfortunately. My butter day are over, because of cardiovascular disease. However, I used to with some sandwiches.

I'm american and my mother always buttered the bread.