gives a nuance of 'ought to', 'supposed to be' or 'should' to sentences, implying high probability.
「はずなのに」is derived from this!
very rarely in kanji, 筈
これは食べられるはずだ。 This should be edible
図書館は静かなはずです Libraries ought to be quiet.
今日、会社は休みのはずだ I'm pretty sure today is a day off for the company.
Very common set expression is はずなのに, coming from はず＋Despite
同じはずなのに、違う。 It should have been the same, but it was different.
来る筈なのに来ない He was supposed to come, but he didn't.