There are two reasons for switching phases in BLDC motors in exactly this way. First, if the rotor and stator magnetic fields were allowed to align perfectly the motor would create zero torque, which is unfavorable. Second, maximum torque occurs when the fields are at 90 degrees to each other. The goal therefore is to bring this angle close to 90 degrees.
However, in BLDC motors, it is impossible to constantly achieve 90 degrees using six-step commutation and the angle fluctuates between 60 and 120 degrees as seen in the following animation. This is owed to the comparably simple nature of trapezoidal control. More advanced techniques such as field-oriented control, commonly used to control PMSMs as discussed before, allows generation of larger torques by achieving 90 degrees between the stator and rotor magnetic fields.