Doing the PPL has the advantage of allowing you to take others with you, as has been said. It also means that you can sign the MR for the daily inspection, something a student isn't qualified to do - unless the rules have changed in the last 10 years.
Those two benefits allow the possibility of doing some of your command flying as overnight trips to somewhere further afield than the typical 3 hour navexs. Ayers Rock, Birdsville races, Wilpena Pound, Kakadu all make for interesting trips with great learning opportunities.
I've also seen one or two cases where a student got stuck overnight** due weather or a fault. It meant having to find licenced pilot able to sign the MR the next day.
The time used for the test can be included within the 150hr CPL so it doesn't necessarily have to add extra flight time to your course. In some schools, the examiner is part of salaried staff so even his/her time may not cost extra.
The experience doing the test can have more intangible results too. A CPL test isn't much more than the PPL test with more polish & skill expected. Doing a PPL first can help reduce CPL exam nerves. After all you will have (partly) 'been there, done that'.
**You do fly with an overnight kit, don't you? Socks, undies, toothbrush, cash/credit card, clothing you'd be comfortable wearing at a restaurant or pub for dinner, and the like.