Like an athlete, I do warm-up (or practice) sheets in the marbling tank, stretching memory muscle and calibrating hand-eye coordination, before diving into "real" tank work.
I'm also testing my bath and paints to see how they are behaving. Like us, a new day means a new personality for both of them and things that worked well yesterday may not today. Warming up is about getting into the groove.
Many times, these practice pieces turn out to be some of my favorite pieces, partly because I have no expectations of them and partly because I don't think too hard about things and just let my creative instinct (and eight years of marbling experience!) take the lead.