Some people have to if they are breastfeeding and going back to work, or if for some reason their baby can't latch. Some also donate excess milk to other babies who need it (eg. if their mums are sick and receiving medication that means they can't breastfeed).
Yes, you can keep the milk in the fridge for a couple of days or the freezer for a month (I think, I'd have to check, it's been a while). It's not as good as fresh but still better than formula (which is in turn much better than nothing, before any defensive formula feeders jump on my comment).