Essentially miscellar water is a a mixture of micro-droplets of oil suspended in water, they tend to be very gentle on the skin while removing make-up quickly and effectively without stripping it of it's natural oils.
However they are a make-up remover and not a skin cleanser in my opinion.
I'm seeing a lot of marketing jargon selling these as a complete, one step does it all skin cleanser and they are not, they are perfectly fine to be use to remove the bulk of your make-up but not to cleanse the skin, only an oil or balm will do that. They are also great for tidying up any makeup mistakes as they wont leave an oily residue behind.
So yes, use them at night but not on their own, don't use them in the morning because there's no point!
Do you have to buy Bioderma?
No, not really, it's expensive and they all pretty much the same. My favourites are the Garnier and Simple one's and they are a tiny fraction of the price and work just as good.