When using Songtrust, setting up your own personal publishing entity will not change the amount of royalties that you will be able to collect. However, some songwriters like to create their own publishing entity for liability protection and tax purposes.
Some writers choose to create a publishing entity to separate their publishing income from their personal income. If you were ever to be sued for copyright infringement or something similar, this would protect your personal assets - like a house or car - from being seized. Additionally, your business would be taxed on your publishing revenue, not you personally.
However, in both of these scenarios, you will still need to create a formal business entity with your local government in addition to affiliating your publishing entity with your PRO.
It should be noted that Songtrust can not offer any tax or legal advice, and recommends that you seek legal council whenever necessary.