It sounds to me like these 'little devils' are:
“These little devils, you know, they all look the same- kinda like one of those English alphabet”, said Grandpa.
An unused staple has two spiky 'horns' like a little devil and also resembles a letter 'C'.
“And they always stick together-in groups; sometimes a hundred in a group.” He continued.
When bought new, staples come connected together in a long line.
“They only leave their clans when they are pushed hard. Then they get really bent out of shape. Even with that condition and only in that condition, son, they become useful”
Staples only leave their long stuck-together line when put into a stapler and you push down hard. This action bends the prongs of the staple so that once it pierces the papers it is pushed through, the prongs bend in on themselves to fasten the papers together - a very useful purpose indeed.
"Billions of them, and their kind, all around us. In homes,shops, offices, buildings, hospitals"
Wherever there is a multi-page document you will probably find a staple holding it together!
And how do they help people 'lose weight'?
By helping people lose paperweights! They 'unite' pieces of paper together and avoid the need for another object to hold them down...
Of course, staples are also used in bariatric weight loss surgery, when the stomach is manipulated with the use of staples to make it smaller and limit its capacity for food - and this was the OP's true intent here...