Parties have to advertise too. Got to keep the press release recyclers fed. All the better if the party polls indicate that some opposition party is not doing well.So the purpose of opinion polling is not necessarily the selling of papers and/or advertising.
You can see how the different media organisations react when their favourite party has a bad poll. They also have to know if the other parties or polls are inaccurate about support levels. Today's RedC poll shows FF and FG losing badly in the 18-34 year old demographic (with the usual high MoE disclaimer) and that seems to be upsetting a lot of FF/FG types. Labour is only at 3% in that demographic but I suppose its supporters are used to that by now. SF 's support range is rather consistent and that's beginning to worry FF and FG to the extent that Simon Harris is calling for an election pact for DBS.
When it comes to the media commissioned opinion polls, they are commissioned in order to sell copies and advertising. Like Astrology, they provide some entertainment.