if you do not have a copy of something in different place, you do not have a backup. raid != backup!, its for reliability (and sometimes speed).
i actually have the local copy, a nightly backup to the nas device, and set of offline drives i keep in a pelican case i refresh a few times per year as a secondary backup.
yes, that is what raid is but that is not a backup. it is making that single logical drive of your data resilient to a single drive failure. if anything goes sideways and you lose 2 drives, you lose 100% of your data. and it does happen. think power supply failure spiking your drives or whatnot.
you dont have to take my word for it, it is well known and well advertised that raid is not a backup.