English 'shall' originally meant 'I owe' — from PIE *(s)kel- (to be under obligation), and the future tense developed because a debt is something that must come due. Legal 'shall' ('the party shall comply') preserves the original obligation sense perfectly.
A modal auxiliary expressing future tense, determination, obligation, or legal requirement.
From Old English 'sceal' (shall, must, is obliged to, owes), from Proto-Germanic *skal (owes, is obliged), from PIE *(s)kel- (to owe, to be under obligation). Originally meant 'I owe' — the future tense sense developed because owing implies something that must happen next. German 'sollen' (shall, ought to, is supposed to) preserves the obligation sense more transparently. 'Shall' is another preterite-present verb
'Shall' originally meant 'I owe.' The future tense meaning developed because a debt is something that MUST come due — what you owe is what will happen. This is why legal language uses 'shall' for binding obligations: 'The tenant shall pay rent on the first of each month' preserves the original sense of 'owes, is obligated to.' German 'Schuld' (guilt, debt) is from the same root