So, let's say you have a basic Flux action that fetches remote data asynchronously, and then dispatches that data to its store. The dispatch is called after the async fetch completes, using a promise or callback. Nothing out of the ordinary here! Just your standard async control flow patterns.
Next, let's say we take this action, tie it to a button's onClick, and then we click it. If the async call takes a bit of time to complete (as they tend to do), that click would feel like it either didn't go through or threw an error, because any components listening to the store would do absolutely nothing until the callback completes or the promise resolves (which is when the dispatch will occur).
If they don't, interactions feel unresponsive, and the app will feel 'broken' in many situations. It will be frustrating to use at the very least!