Eliminating all unnecessary pressure
When you are on a franchise like the Celtics, there is always going to be another level of pressure that most teams cannot match. There is a different kind of standard in Boston, and that means struggling or not rebuilding properly puts more pressure on the team, with a much shorter leash.
During this rebuild, however, the pressure has been nonexistent. The organization did a great job of allowing the team to rebuild properly, but the looming threat of the Warriors made that easier.
The Celtics knew they had to be even more patient, and give all their pieces as many opportunities as possible, because rushing anything would make it impossible to catch the Warriors.
Over the past few years, there has only been a handful of teams with legitimate pressure to win the title, because expecting any team to beat the Warriors has been borderline delusional. That has allowed the Celtics to accomplish things in a season and be successful, despite not getting that championship.
Taking away that pressure has given both Danny Ainge and Stevens an added level of freedom and security. They have known that failing to get a championship has not been a condemnation of their rebuild, and that standard may not have existed without the inevitably of the the Warriors.
It is difficult to gauge how patient he Celtics would have been without the Warriors super team, but you can be confident that it contributed on some level to the culmination of their rebuild.