Advanced Target Priorities 1.1 - allows you to choose what your units are shooting at. The applications are endless. But in particular: you can set your navy to prioritize destroyers so they will shoot destros instead of frigates. (Or cruisers, or engineers, if you want to pick off engineers around a navy factory)
You can set your ACU to target units, so you won't waste shots on the enemy ACU. Of course you can also prioritize the enemy ACU if you want. Depending on the situation. Or you can tell ACU to prioritize engineers if you want to pick off a few.
You can set mobile arty to prioritize factories while your tanks prioritize enemy units, so you can do max damage to the enemy
When you are telemazoring, and trying to teleport away, you can use target priorities to decide what your ACU is shooting at. Normally when you are teleporting you can't change targets because you can't give new orders to your ACU without canceling the teleport. So you can switch between targeting power and PD or whatever.
There are just a huge number of possibilities with that mod.
Selection Cost UI - this is mostly useful when analyzing replays. When you select units, it shows how much the selected units cost. This makes it easy to compare things like "how much total mass did I invest into my army, and how much total mass did my opponent have at the same time?" or "how much mass did I throw away when I lost this army group?"