Generally speaking, fixed-term tenancies give more certainty to both tenants and landlords as neither party can terminate the tenancy early (or at least terminate it early without some repercussions). In contrast, periodic tenancies can be terminated by tenants (by giving 28-day notice or landlords (by giving 90-day notice and having a specific reason to terminate).
We have a blog article on this topic where you may find some more information:
What is a periodic tenancy? Periodic tenancy vs Fixed-term tenancy
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