No problem of course there may be some cases where you may want to execute ajax synchronously but I honestly don't remember ever doing that. Out of curiosity I checked the video you mentioned https://www.youtube.com/watch?v=dRZxQY2RgUA and the guy realy says:
This "async"... I wish I could explain it in more detail..uhm it's just data thing and I don't fully understand it
Short explanaition. When you set
async: true your code doesn't wait for the response of ajax call and your application keeps running while the data is being requested from a remote server in the background.
When you set
async: false the code executions stops until you get a response from a remote server. If JSON (or whatever else) you're requsting is a large file or simply your internet connection is slow it means your app freezes and becomes unresponsive until it finishes fetching data. Setting it to false would only make sense if the data you're requesting is crucial for your app to keep on running. The guy in the video also says about "async" that it
works when it's false
Not true. When it's false its behaviour is just more predictable and easier to debug.