Why do some coaches raid their former clubs?

[Heatman]

I keep observing this with some coaches whenever they leave a club to sign for another with how they keep going back to poach players from their previous club to their new one. Ten Hag did it with his Ajax players after signing for Manchester United. 

Can't they do without their old players? 

[killamch89]

Familiarity is the primary reason as that player already has an idea of what the manager expects of them and how he wants the team to play. Thus the adaptation time should be much shorter. Of course, it doesn't always work out.

[Knight Barida]

It is all about familiarity and trust. They achieved a lot with those players, and they have this belief that when they bring the player to their new club, they will achieve greater things. So, that is basically what I see playing out in most of those deals.

[Heatman]

For me, I think it's because they don't have what it takes to attract other top players from other clubs and that's why they go for their previous club players. 

[killamch89]

13 hours ago, Heatman said:

For me, I think it's because they don't have what it takes to attract other top players from other clubs and that's why they go for their previous club players. 

I mean, that could be one of the reason but the primary reason is usually familiarity.