At its root, a preliminary notice has a specific primary purpose: to provide top of chain parties with important information about who's performing work on their job site. This can be crucial considering construction projects require a joint effort between many separate parties. When every party on the job sends preliminary notice, it's easier to understand how payments will wind their way down the payment chain. With this information, it becomes infinitely easier to stop payment disputes before they start.
As a secondary purpose, preliminary notices are often required by statute, and if not sent, the party who should have sent notice will lose some rights related to recovering payment. This is an interesting situation though - because by sending preliminary notice and opening lines of communication, payment disputes become less frequent. Meaning, by sending preliminary notice, the sender makes it less likely they'll ever have to utilize the very rights that a the notice preserves.
Sending notice at the start of the job provides a win-win scenario for contractors and their subs and suppliers. For contractors, these notices create a full picture of a job and make it easy to be sure payments go where they need to. For subs and suppliers, these notices make sure the contractor knows they're there - plus it preserves the right to later make payment claims, if they become necessary.