The crux of strategic voting is that you know your preferred candidate is not likely to win, so you have to decide whether to vote for them anyway or vote for another candidate with a greater chance of winning. It’s a critical decision in this election where the Green and NDP platforms are quite similar in many ways and the Liberals have a number of positions opposite to both, so a Green supporter who is in a riding with little chance of winning might be tempted to support an NDP candidate who is running a close race with a Liberal.
This is a complicated and difficult situation for any voter to be in, and it does not happen with proportional representation because in proportional voting systems, the crux described above can never happen. In a party based proportion system your vote will always count towards electing your candidate or an alternative candidate from the same party. In STV your vote will always count as you choose in your preferences.