Yeah, it's to set you up on a prepaid rent cycle.
And the absolute most frequent time people neglect to pay their rent - you already said it - the last month they intend living in your property.
First month rent
Last month rent
Additional one month rent as security deposit (compensates for the "it's Christmas" excuse, early lease breaking, etc.)
Additional one month rent as damage deposit, and......NO PETS!
You gon pay me!
If you ain't got the Bennie's to move in, you cant afford the rent to stay...and I cant afford for your dead @$$ to live on me for free.