"Useful God" is actually a bad translation. It misleads the reader to think about "usefulness".
In fact, the character "yong 用" conveys the idea of "focus". The focus of the chart is the distinct feature of the chart. The particular deity/god that is responsible for that feature is the Yong Shen - the deity we focus at. Obviously a chart is good if the Yong Shen is healthy and strong. A bad luck period will have this Yong Shen attacked by the luck pillar. However, if the Yong Shen is protected in the natal chart, then the attack may be dissolved.
How to identify the Yong Shen in a four-pillars chart is simple and not simple. Some people say scornfully that Yong Shen is unimportant. Well, it simply means that these people have no focus when they read a chart.