A contol's refnum allows you to have access to the control from anywhere you want - another loop, another VI, even another computer.
The advantage of using programmatic control is when you want to manipulate your UI, control things remotely or control things dynamically. The main disadvantages are a performance hit which comes from using property nodes and the breaking of the dataflow. Using property nodes to change the values of control remotely can be very hard to debug, because unlike local or global variables, you have no way of searching for it.
You can start by reading chapter 17 of the LV user manual (Help>>Search the LabVIEW Bookshelf) and by having a look at this tutorial [broken link removed] about class hierarchies in LV.
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