A citizen has just one vote. The voter expresses preferences by using the ballot paper to instruct the returning officer as to what to do with that one vote. The number 1 says, “That’s my preferred candidate”. The number 2 says, “If my no. 1 cannot be elected or doesn’t need my vote, then give it to number 2.” And so it goes.
At every election some fool will argue that later preferences are to be opposed for fear of electing candidates a voter might oppose. That’s simply not true.
If a voter has expressed preferences for a small list of desired candidates and then has absolutely no preference as to which of the remaining are elected, then it makes sense to stop. However, the application of a little thought might reveal some preference as between the remaining candidates, e.g. a voter might prefer a woman over a man from the same party or a candidate who has expressed a mildly different view from the others remaining.
Moreover, if the voter really has no preference whatsoever between the remaining candidates and stops at, say, number 3, that voter has no further effect on the outcome either to oppose or to elect someone from the remaining candidates. They simply say to the returning officer, “I don’t care beyond my number 3. At that stage count me out.”
Say there are eighteen candidates. Sensible advice to the voter would be as follows. Give your 1st preference to the candidate you most want elected. Give the candidate you least want elected your number 18. Now list the remainder from 2 to 17. It might be hard to decide between some of your lower preferences but at least you can say that you prefer them more than number 18!