It's basically to stop people using properties as if they were 'holiday lets'.
Holiday lets, and other short-term rentals, tend to charge higher rents than other properties do. That's because the landlord risks ending up with lots of periods without any tenants.
With a 'normal' rental property, the landlord needs to have the security of knowing that a tenant isn't going to vanish after, say, just one month of living in the property, leaving him/her without a tenant (and thus without any income from the property for a while).
If that kept happening, and it took a month each time to find a new tenant, a landlord would only get 6 month's rent per year from a property.