Because when your ex leaves, you might try to fill this void in a rebound way which is not satisfying for you long term.
Keep your options open instead.
Light intimacy is great but think twice before having intercourse as this usually creates very strong binding and often naturally leads to commitment.
As soon as you have sex, expectations or drama with this new partner might occur.
If you are still sensitive and vulnerable after your break up, this might ad stress and challenges rather than giving you space and energy to recover
Enjoy your freedom for a while instead.