say you where managing at Chelsea and then you went to Fulham and you wanted to take John terry with you, he may not be so keen on the idea so if you put that clause in his more inclined to go to Fulham so he can stay with you but if you suddenly leave after 6 months his not stuck there and can move on if he wants
If you have a good relationship with a player, if you take a job in a higher league eg you play in Ligue1 and take a job managing Inter or Barca and they are wonderkids they can and will usually follow you if you want them