The upside is that you always have your account "active" and ready to go. There is no friction to playing a video, browsing the catalog. Just hitting play and you are a paying customer again. If you are going to be charged for two years there is definitely incentive to cancel your account, adding friction to any decision to resubscribe.
The downside is likely that it adds mental friction to that first video every month. It takes it from "well I already paid" to "is it worth it? I'm going on vacation for 2 weeks and won't watch much, maybe I'll skip this month" which negates the mental relief that "unlimited" monthly subscriptions rely on.