Phase 1 or interceptive treatment is a relatively short orthodontic treatment (6 months to 1 year). It takes place at age 6-10, in mixed dentition before eruption of all permanent teeth. It allows to create space for next 12 permanent teeth erupting at age10- 12. It also prevents jaws from growing side ways. It’s also the right time for stopping habits which can deform the mouth (such as thumb sucking or tong thrusting).
Phase 2 takes place at age 11-13 or later. Braces can settle teeth in their final balanced position.
Phase 1 or interceptive treatment is not necessarily needed. It’s up to the orthodontist to see if really save treatment time on phase 2.
However interceptive treatment, being done in mixed dentition, never avoid a treatment in permanent dentition ( phase 2). It only makes it shorter.