Making It a Thing こと

ピアノを弾くこと packages the whole action “playing piano” into a single noun, letting it slot directly after です, exactly the way ことができる wraps a verb before できる. This is the same こと, doing the same nominalizing job in a broader range of positions.

Wrapping a whole fact in こと lets you attach a feeling to it directly, ことが嬉しい, “the fact that ~ made me happy,” ことに感動する, “was moved by the fact that ~,” a natural way to react to an entire situation rather than a single object.

Attached directly to a noun instead of a verb, XのことでYou get a slightly softer, more roundabout way of referring to a topic, 試験のことで心配, “worried, regarding the matter of the exam,” than simply 試験が心配, “worried about the exam.”