I've seen some leases where it explicitly states that tenants aren't allowed to engage in illegal activity. Is there a point to this or does it go without saying? Here is an example.
Saying that illegal activity is not allowed means that illegal activity is a breach of the lease, so it allows the landlord to evict the tenant if there is illegal activity.
The landlord/tenant law with which I'm familiar already specifies that certain illegal activities are grounds for eviction, but it is possible that there are other illegal activities that would not be grounds for eviction under the law, but could be grounds for eviction if mentioned in the lease. Finally, there are probably some very minor illegal activities that would not be grounds for eviction no matter what.