What should ECRs and PhDers consider when choosing a conference? Purpose, cost, and motivation

For many early-career researchers and those studying for a PhD, settling on which conference(s) to attend can be a tough and fraught decision. So what is the most important thing to consider? Pat Thomson suggests three answers to this question, covering why you believe you should go to an event, whether it represents value for money, and whether or not you truly want to go.