If a team consists of many members, pairs tend to get friendly. Like a courtship. If a pair starts to get along very well, then they prefer to only pick each other.

This leads to specialities and can cause specialization. Example John and Peter always work on the database layer code and get it done really well . Hence the team starts giving all the database tasks to them starting to cause bottlenecks. Everyone who needs database scripts are now waiting for John and peter to get it done.

Promiscuous Pairing is way to avoid this.

Pairs change on a daily basis. Tends to keep the knowledge well spread across the team

A random pair draw at the daily meeting is a way to pick these pairs.

This could cause other effects as developers tend to like some and not like others . Promiscuous Pairing forces the member to get over thier issues and can cause unforseen tensions and a drop in efficiency.