What are the possible uses of drill vs. combat? e.g. for units like rifles and infantry.
There are lots of discussions on this topic - I don't think there's anything definitive, but enough information in Strategy and Tips that you can form your own opinion.
That said...
The primary motivation for first strikes is maintaining a set performance level - in other words, continuing to do well in combat even though you haven't had an opportunity to heal.
The two circumstances where this is most important are defense - where you may be on the receiving end of several attacks on the same turn - and when you are capturing territory, and need to move quickly to the next target.
Classic example of the first - archers defending against the hoards in a Deity/Always War/Raging Barbs/Pangaea doomfest. Another - using a defensive piece to hold a bottleneck. In both cases, you are worried less about beating one strong piece than many average ones.
For the second example, you're generally looking at trying to capture cities quickly. Disposable siege units are weakening garrisons, and then you want the stack pieces to kill without taking damage, so that you aren't weakening your force leaving pieces behind to heal.
In other words, you are using the first strikes as a poor man's version of March. So the right comparison, if somebody want to run the numbers, might be Drill IV vs Combat III + March.