I want to get properties on the table, but not complete the set. I am trying to avoid having a complete set stolen. I realize by keeping same color cards separate, they could be taken in a forced deal or sly deal, but when it is my turn, I can arrange my properties how I want to collect rent or if I get three complete set, arrange them on my turn for the win. But by keeping the same color cards separate, at least I won't lose an entire set when the deal breaker is played.
If you have enough to make a set, then you have a set.
From the compiled FAQ on BGG:
If you have more properties of one color than is needed to make a complete property set, you must form as many complete sets as possible, and group the remaining properties of that color together.