Very, very subjective. To sell anything to anyone you need to understand the context in which it will help them.
What pain points are they having? Those are the things you want to target with your pitch. Much easier to sell water once you know the client is thirsty.
Having said that, PMI have a good whitepaper that outlines some of the points you could pitch and it comes with some helpful statistics: https://www.pmi.org/Business-Solutions/~/media/PDF/Business-Solutions/Value%20of%20Project%20Management_FINAL.ashx
Some quotes from the paper:
“Good project management discipline stopped us from spending money on projects that fail,” says Ron Kasabian, general manager at global IT giant Intel, Folsom, California, USA.
Ninety percent of global senior executives ranked project management methods as either critical or somewhat important to their ability to deliver successful projects and remain competitive, according to the Economist Intelligence Unit survey.
Lots of other quotes in there depending on how you wanted to focus your pitch.
Most will contain some variation of the following:
- Reduce risk
- Increase project success rate
- Decrease project cost
- Increase velocity / productivity
- Improve quality / satisfaction / uptake of outputs
That should give you a good start!