Lower Suspension Arm replacement

[ap69]

i am going to attempt to replace both lower suspension arms .Looking at the Haynes manual and as a novice mechanic it states to tighten both the bolts to a specified torque and then a further 60 deg followed by a further 15 deg...Why is this done and not just 75 deg in one go?..Anything to look out for when attempting this work?

Thanks

Alan