Use ～ことだろう in written or formal spoken Japanese to voice a confident inference about something you can't directly verify — how a client feels, how a market will move, how much effort a team put in. It carries more emotional weight and rhetorical polish than ～だろう alone, so it suits reports, speeches, and carefully worded email rather than casual chat. It pairs naturally with さぞ, きっと, and どんなに (the latter giving an exclamatory "how ~ it must be!" reading).