For me, they have worked with each other for a long time and will know each other inside out and what their strengths and weaknesses are. I’m sure they will have also developed a way or dealing with the difficult conversations if one is underperforming etc.
I can understand the jobs for the boys thing but every manager brings their own team, usually people who have been with them across many jobs and many years. Look at Allardyce bringing Sammy Lee. Lampard bringing Cole etc. and even more so, Carlo bringing his son.