For those who don't know .... like 411A (who presumably also drinks cows' milk straight from the teat .....!)
One expresses breast milk in much the same way as one gets cow's or goat's milk - either by hand (which is frankly about as much fun as milking a cow for either mum or dad) or by pump.
It serves the same purpose - you can use it later on. This is particularly valuable if your baby was premature, and in an incubator, so you can't feed it yourself, or if mum is going to work or out for the evening and wants somebody else to feed baby, but wants to keep up your production of milk (which drops off if the baby feeds on formula), or to ensure that baby gets the best all the time.
I never had the nerve (as an adult!) to taste a large sample (either expressed or "on tap" as it were), but in small doses, it was sweet and not as "heavy" as cows' milk. My children seem to have done OK by it.
Breast is best!
There is a lot of stuff about childbirth that, frankly, it's best not to know about until you have to ... for the sake of the continuation of the human species.