One obvious reason for res judicata is that it provides finality to a matter. If a person could re-litigate the same matter over and over again, the parties could never move forward. They would be in a perpetual state of limbo. Clearly society works better when matters are resolved and the parties can move forward with their lives.
As mentioned, an approved trustee's accounting is entitled to res judicata for the matters it addresses. The following is an example of how res judicata plays out in a hypothetical situation.
A trustee files a petition to settle an account and approve their compensation under Probate Code § 17200(b)(5),(b)(9). In the petition, the trustee asks for $6,000 as trustee compensation. The trustee, a lay person, states that they expended 300 hours administering the trust and appends hand-written time sheets to the petition. Their billing rate is the reasonable sum of $20 an hour.
The trustee provides notice to all beneficiaries of the hearing on the accounting petition. No beneficiary however attends the hearing and the judge approves the accounting, which permits the trustee to pay themselves $6,000 from the trust.
Months later, a beneficiary files a petition to instruct the trustee to pay back the $5,000 to the trust per Probate Code § 17200(b)(6). The beneficiary believes that the trustee exaggerated the amount of hours they worked on the trust. The beneficiary cannot believe that it took 300 hours to administer the trust. The beneficiary believes that 50 hours is an acceptable amount of time.
The beneficiary's petition will be denied because they failed to object prior to or at the hearing on the trustee's accounting. If the beneficiary can re-litigate the issue of trustee compensation, they would get two bites at the apple. First, the petition that the trustee filed under under Probate Code § 17200(b)(5),(b)(9). Second, the petition the beneficiary filed under under Probate Code § 17200(b)(6). Since res judicata says that a litigant only gets one bite at the apple, the beneficiary's petition will be denied. Cue fog horn.