What are some reasons to connect capacitors in series?

Another reason when done in production designs is to reduce your bill of materials (BOM). If your design has loads of 100 nF caps but needs one ~50 nF, it is often cheaper to use two 100 nF's in series due to the quantity you're buying the 100 nF's in, and also reduces pick/place setup time.

In an automotive application I've seen two ceramic capacitors in series to increase safety against shorts. In the extreme case a short could start a fire, and I heard that had happened at least once.

I have only seen it done to increase voltage. On some power supply front-ends (AC/DC conversion) with a voltage doubler the capacitors are in parallel at low voltage and in series at high voltage. This works out well since for a constant power out the current is double at the lower voltage.

As you mention balancing resistors are required.