The primary reason for this practice has little to do with political parties wanting their voters to follow their preference instructions, beyond voting for their party.
The primary reasons for HTVs is historical, and related to party exposure. Election campaigns in the United States and the United Kingdom do not focus on handing out pieces of paper to voters on election day, instead focusing on get-out-the-vote efforts. There are efforts made to put up signs and have a presence at polling booths, but the primary aim of the campaign is not to hand a particular new piece of election material to voters as they come to vote. Even in Ireland, the country closest to Australia using a system of preference voting, I don’t believe they hand out how-to-votes.
It was previously the case that it could be quite difficult to fill out a ballot paper without the help of your political party. For much of the twentieth century, all mainland state and federal elections were held using compulsory preferential voting, and with no identification of a candidate’s political party on the ballot. Voters for a major party would need to number every box, and may not be able to identify little-known candidates for such parties as the Communist Party. A how-to-vote would allow voters to number every box, even if they had no idea about a number of the candidates.
This is less of an imperative now. In state elections in New South Wales and Queensland, optional preferential voting means that voters can cast a formal vote without numbering many boxes. In most electorates the major parties advise voters to “just vote 1”. In other states, and in federal elections, party labels on the ballot paper make it easier for voters to number every box. Ballot papers also make it clear when voters need to number every box. Of course, many voters may lack the confidence or intelligence to work this out for themselves, but it remains the case that it is much easier to vote.
The main reason why political parties focus on handing out how-to-vote cards is exposure. It is the last opportunity to have contact with voters, and everyone else does it. It becomes a kind of arms race, with parties in marginal seats focusing on trying to have more workers than the other party, even if they are not all necessary to reach every voter. While it wouldn’t have a serious impact on elections if no party were to hand out cards, it would likely be detrimental for a political party if they stopped doing it unilaterally.
I’m sure it is true that a secondary reason for handing out HTVs is to ensure all of your voters do cast a formal vote. This would particularly be an issue in areas with low literacy rates or language diversity, where many voters may not be fluent in English. A how-to-vote can allow them to copy out their vote without understanding the instructions and can also help with confusion in states where the rules vary between federal and state elections.
If a party is one of the top two polling parties in a district, their preferences will not be distributed under any circumstances. Except for rare three-cornered contests such as Balmain, it is usually clear who will come in the top two well in advance, so the major parties know their preferences won’t be counted.
In some cases, preference decisions are part of a deal with another party. A party might swap preferences between a district where they won’t win and a district where they could win, or for preferences in the upper house. Even if preference decisions are made based on principle, they are usually not the primary motivation for handing out HTVs.
Discussions on preferences often dominate airtime for smaller parties, and in some places can be the only media a small party can receive. Rather than being an opportunity to influence politics, it can often prove a distraction, preventing the party from discussing policy issues that may have motivated them to run in the first place.