Is a probate always necessary?

No, if the deceased person held property in a joint tenancy with someone else, or as survivorship community property with a spouse, or in a trust, payable-on-death or transfer-on-death accounts, those items need not go through probate.   However, California will likely require a probate whenever someone dies without a will or trust, with only a will, without a properly drafted trust, or if there is a dispute as to the validity of those nonprobate transfer instruments, e.g., a defective joint tenancy deed.