The casinos like to corral their bettors according to how much they bet. One reason for this is the higher limit tables have fewer players so the big bettors get in more hands per hour. Another reason is that it is said players like to be around other players of similar bet size. If a player wanted to bet $1000 at a $5 table it might make other $5 players at that table feel nervous or uncomfortable. A third reason is it is a preventative measure against cheating.