The American election followed roughly a year after the Canadian election (2016 versus 2015). Facebook claimed that their platform had an significant effect on the outcome of the Canadian election via targeted advertising, and were promoting themselves as allowing customers to essentially buy elections for not very much money by targeting messages at very select groups, pretty much down to the individual level. This was part of the sales message they were taking to potential political advertising customers, and Canada was being cited as an example of its effectiveness.
Now I happen to think that Facebook's claims were rubbish, and the CBC published an analysis piece which also rubbished Facebook's claims with data to back up the CBC's story. What had happened was that there was a government who had long outlasted their welcome and had in the course of the election reminded people of the very worst characteristics of themselves and of the moral depths which they would plumb. Social media on the other hand had an insignificant influence on the results of the election.
However, we have to also admit that to someone outside of the Canadian political currents, Facebook's sales message may have seemed convincing.
Now to consider another aspect of the equation, American money from entities close to the American government of the day was used to try to unseat the Canadian government during the election. It is illegal for foreign money to be used in this way, but the funds were laundered through various front organizations to conceal its origin. The money wasn't funneled directly to opposition parties (who would have been extremely unlikely to have touched anything that dodgy), but was instead sent to front organizations who were dedicated to using advertising and agitation targeting marginal seats to defeat the government.
After the election what had happened leaked out. The Conservatives cried foul and demanded an official investigation into election interference. The Liberals sat on the notion and blocked any investigation from being conducted. While they weren't involved in what happened, they could only lose from anything which called the legitimacy of their victory into question. And any investigation which ended up pointing a finger at the Americans would probably result in things turning out very badly for Canada in a number of different ways.
Now imagine yourself sitting in Moscow observing all this. You have Facebook salesmen knocking on your door telling you that you can buy elections via Facebook ads and showing you their studies which they claim prove it. You have reports telling you that it is possible to interfere in foreign elections and get away with it provided the money can't be traced directly to your government. The victor will not only sit and take it, he will have incentive to actively block any investigations.
It doesn't take a lot of imagination to see that putting the two together could be a low risk, high reward gambit. Also recall that Americans with close connections to the Clintons had not only involved themselves in a Russian election in the 1990s, they had openly bragged about it later.
To go back to your question of "why do they bother?", it probably seemed like a very plausible idea at the time given the events which took place in the year before. They just needed to stir the pot and the Americans would be spending so much time tearing into each other that they would be too busy to bother anyone else.
It all sounds good from a purely theoretical perspective. However it suffers from three major failings. The first is that Facebook were full of shit. The second is that the governing party in Canada lost for the reasons that I outlined above, not due to foreign interference in the election. The third is that when the Russians tried it in the US, they were shit at it.
Legislation has been tabled in Ottawa recently which is intended to try to prevent interference by outside entities in the next election.