Is there any use in archiving graded 8-bit footage as 10-bit file?
Yes, you will lose information from the grade if you save as 8-bit. 

Here's a thought experiment to illustrate. Say your original red level for a pixel was 128 out of 256 (00001111 / 11111111), and then in Resolve you grade it (using the internal 32-bit floating point colour in Resolve) to be a little bit brighter, say 128.25. Now you'll need to save it in 10 bits  or more to keep that information (e.g 000111101 / 1111111111 where the two right-hand-side bits 01 / 11 = 1/4 = 0.25). If you saved it as 8-bit it would have to be rounded to 00001111, meaning it would go back to being 128.

The upshot is, if your new colours don't fit into the 256 values per channel of 8-bit colour they will get rounded to the closest fit. So while you don't gain anything from up-depthing (that's a thing now, because I just invented it) ungraded footage, you do gain by not collapsing the higher-bit-depth graded footage back down to 8-bit.