There used to be just two grades of warrant officer in the Canadian Army. In 1939, a third grade was added, but it was shortlived. The bare crown represented a Warrant Officer II Class, the coat of arms a Warrant Officer I Class.
When the third grade was added, the bare crown became WO III, and something was needed to distinguish him from a WO II - so, I suspect, the wreath was added.
The WO III fell from disuse about a year after it was introduced - 1940, but when the rank system changed in the 1960s/70s, the three WO grades were again introduced. The rank of Staff Sergeant was abolished, and the bare crown came back into vogue; today we call him a Warrant Officer, while the wreath and crown is a Master Warrant Officer.
Wreathed crown insignia also seems to have been used in the First World War era army, introduced in May 1915. Up to that point, a bare crown signified a "(regimental) sergeant major" and three chevrons and a crown a company sergeant major. Four upside down chevrons and a crown was a regimental quartermaster sergeant.
In May 1915, the wreathed crown was added, as well as the coat of arms, to the variety of insignia to better distinguish men holding various positions. The RSM then wore the coat of arms (and a Sam Brown belt), the CSM/RQMS wore a wreathed crown, and company quartermasters wore three chevrons and a crown.
http://www.canadiansoldiers.com/ranks/nco.htm
Confused yet? More pictures at my webpage, above.