What is important about Polonius’s instructions to Reynaldo as contained in the lines “And thus do we of wisdom and of reach, / With windlasses and with assays of bias, / By indirections find directions out”? How is this reinforcing a theme in the play?

In this passage, Polonius is sending his servant Reynaldo to spy on Laertes in Paris. He instructs Reynaldo, among other things, to find out where Danish expatriates are staying in Paris and then to ask them in roundabout ways what Laertes is up to, without revealing how close Reynaldo is to Laertes. He wants Reynaldo to draw people out without them suspecting anything, so that he, Polonius, can get an accurate report of Laertes's activities.
Having minutely instructed Reynaldo in underhanded ways to find out what Laertes is up to, Polonius sums up his methodology: one should by indirect means and deception find out what is going on. Polonius has more trust in this method than in straightforward ways of gaining information. When he uses the phrase "assays of bias," he is referencing the game of bowls, where the bowl is moved in a wide circular pattern instead of a direct line.
This reinforces the play's theme that the Danish court, as represented by the important courtier Polonius, is corrupt and underhanded, plotting behind people's backs rather than being upfront and honest.
http://www.shakespeare-online.com/plays/hamlet/examq/three.html