Under hourly billing, every phone call is a billing event. Every angry letter from the other side costs you money to read and more money to answer. The longer a case runs, the more it costs, which means the billing model itself quietly rewards conflict.
A flat fee turns that off. The fee is the fee. You can call me when something frightens you at 9 PM without doing math first. And when the other side files something designed to run up your costs, your fee doesn't move. Answering it is already inside the flat.
A flat fee has its own temptation, and you should know it: a lawyer paid up front could be tempted to do less. That is what the defined road is for. Every stage the fee covers is named in writing before you pay, so you can hold me to it.
What the flat fee buys is a defined road: the petition or response, the mandatory disclosures, discovery, negotiation, mediation if ordered, motion practice, and an agreed decree if the case settles, which most do. The one thing it deliberately does not include is a contested final hearing where witnesses testify. If your case turns out to be the exception that needs one, that stage is priced separately, in writing, before it starts. You decide with the number in front of you.