To be honest, when Sergi played the few minutes he had this season. He never dissapointed. Wasn't too flashy either, but did the job. I really don't want to see him go.
It is a sign of respect and gratitude towards the retired player. Atleast, I think it is the common way people think about this.
It's like : No one can ever replace you so you're number will be forever remembered or sumthin like that.