In parliaments, before voting on some bill, law, or policy, there are debates, where supporters and opposers highlight various thoughts. Then the voting is conducted.
Now, there is always the "party line" (some countries even have the party whip) which makes the parliamentarians vote in a certain way. That renders the debate meaningless: it cannot convince the other side, it can not harden their stances, and it cannot change the minds of others.
What then is the purpose of parliamentary democratic debate? Why not just skip directly to the vote?
Example 1: We want to increase the salary for Parliament members.
Debate.
Vote: YES!
Conclusion: PASSED.
Example 2: We want to increase taxes.
Debate.
Party in Power: YES. Opposition: NO.
Conclusion: PASSED.
We can frequently see news reports which claim that the bill will go through (due to the party positions and numbers) even before the debate. That renders the debate meaningless.
(I am excluding cases where we have fence-sitters, who may decide based on the debate.)