Growing outside can be less expensive sure but almost all pot grown outdoors till fully ripe has mold or other debris on it that clean indoor does not have for the most part. Never mind rippers! And heavy rain and wind that can and will occur every year in my area which makes it a big waste of time some years.
Here is some reason to tie down a marijuana plant.
Increases marijuana yield
Security (Do Not Get Busted)
Promotes marijuana plant branching
Increases marijuana bud sites
More sun coverage for the whole marijuana plant
All these are positive attributes. Creating a stronger, healthier, more potent marijuana plant.