是…的 frames one detail of a known past event (我是昨天到的, it was yesterday I arrived). 會…的 frames a whole forecast and seals it as certain (他會來的, he will surely come). 是…的 looks back and picks a detail; 會…的 looks ahead and closes off doubt.
Bare 會 forecasts (明天會下雨, it is likely to rain). Add the 的 tail and the forecast is sealed as settled (明天會下雨的, it will rain, count on it). 會 reads the odds; 會…的 stamps them shut.
stating a bare guess as a sealed certainty: 也許他會來的 (a hedge cannot ride the seal) → 也許他會來 (drop 的) or 他一定會來的 (commit to it)
negating with 沒: 他沒會來的 → 他不會來的 (surely won't)
treating it as past focus: 他是昨天會走的 → 他是昨天走的 (是…的, past detail) or 他明天會走的 (會…的, future forecast)
English fuses prediction and certainty in one 'will', so learners drop the sealing 的 and lose the closed-off, no-doubt tone the pattern carries.