People who are (say) Left-ish but not full blown supportes of BigLeftParty have the choice of voting for the major Left Party or an alternative left faction .. which is a wasted vote in a tight FPtP contest.
Worse a not BigLeftParty vote is one that effectively supports the major Right Party as it lowers the count for BigLeft.
Preference systems, Ranked Systems (there are variations) allow for direct first preference votes for a variationLeft Party with secondary fallback BigLeft preferences.
The direct support for not the main platform shows and causes deals to be made, the votes that fail to elect a variation still fall through and support the LeftWing while allowing the growth of alternative views.
Ranked voting fosters a healthy ecosystem of alternative smaller parties which can then grow, take seats, and help better represent the population at large with a diverse range of views, not just the BigTwo which end up as slightly variations of things not many support.