In my next build, I expect to use a 1TB M.2 drive. I haven't decided exactly how to partition it, but I'm leaning toward 600GB/400GB.
Why partition it? Ease of backup. The bootable C: partition will contain executable programs and all the configuration stuff that goes with it. The D: partition will contain data that loads fast (other data that's archival or otherwise doesn't need to load fast goes on a hard drive). I can easily back up the D: partition and the hard drive directly to an external hard drive every few days, or as needed. The C: partition, whose content changes less often, can be backed up less frequently with a drive imaging program (Terabyte Image) that can restore the system if the drive fails.
I've partitioned my system drive that way for years,