Confused on Artifacts

It really depends on where you are in the game:

At Early Game, flat stat pieces from the bottom row have a use because they can give more stats than percentage pieces for low-level toon.

By Mid-Game and through Late Game, flat stat pieces from the bottom row should be autosold because percentage pieces are always better.  

At End Game, a Flat Stat piece from the bottom row may have value again depending on the substats and the set.  For example:  A Flat HP set of gloves with speed, res, acc & Def % in Supersonic might be just the ticket to finish the build on your Supersonic support.

As far as where to use the newer 9-piece sets?  The answer is everywhere.  They allow you to stack more stats than regular gear because you can bring accessories into the mix, allowing you to create 2-5 piece builds to mix/match with regualr artifact sets to increase stats.

Once you are fortunate enough to have 6-9 piece builds with any of the 9 piece sets, they each offer their own unique benefits.