I'm going to take an extreme case here but bear with me.
Lets assume that you have Borax that is 10% impure, or onlly 90% pure! Lets also assume that the impurity is Sodium Chloride.
A saturated Borax solution will have less Borax in it than expected due to the common ion effect. This causes the less soluable of two sodium salts to precipitate more than predicted.
The 10% Sodium Chloride would become MORE saturated in solution and therefore the impurity level would be increased.
So, your saturated solution is less pure than the starting example.
This hypothetical example shows what you can do to yourself if you don't know the basic guidelines of what you are purifying or how you are purifying it.
Now, if there were a less soluable sodium salt, such as Sodium Aluminate (IIRC), it would concentrate in the solids that remained behind. So the precipitated Borax would end up with more of the alumiate salt.
Either way, you can make things worse.