Not all trust grantors need to fund spendthrift trusts for their heirs, but they can be ideal when the heirs have proven themselves to be less than fiscally responsible.
For instance, suppose you have two adult children -- a son and a daughter. Your daughter has always been the epitome of responsibility and has adeptly navigated through life's shoals. There is no evidence that she would need the micromanagement a spendthrift trust provides.
Your son is another case entirely. Of course, you love him as much as you love your daughter, but his transition from adolescence to adulthood was littered with drug addiction, burglary arrests and children born out of wedlock that he only haphazardly supports.
To put them on equal financial footing after you have passed away would likely result in disaster for your son -- or even his death if he's still using.
You can share your concerns about this predicament with your California estate planning attorney. They can then help you determine how best to provide for both of your adult children in a way that won't place an undue burden on either.
One important point to address is who will be the trustee of the spendthrift trust for your son (if you go this route). It's generally advisable to choose an independent trustee to distribute funds to your heirs rather than appointing a sibling to oversee the spendthrift trust.
Doing that can eliminate the possibility of a rift in the relationship over power struggles involving money -- a legacy no one wants to leave to their heirs.