Joyce, the primary reason why a foreigner should always use an attorney before entering any contractual agreement, be it business or a property lease for example, is that the attorney, and only the attorney can argue the contract in a court of law should that become necessary later on. To the extent that an attorney is involved in the process of drawing up the legal contracts, that attorney is bound to defend its legality and the legal rights of his/her client in court should that become necessary.
Yes, the services of a notaris will also be required, and indeed, as you say, the attorney will use a notaris for certain aspects of the required filings, witnesses, etc. to a legal contract, but, the ultimate and final legal recourse the foreigner has, should things run amok, is his or her attorney.
Yes, for 99% of the daily legal affairs (non criminal) that Indonesians face in their lives, a notaris is most assuredly sufficient, but for a foreigner, a notaris alone is not sufficient for their legal protection.