为什么在web应用程序中使用消息队列

When developing my web application using Django, I faced a problem, when I call some functions locally they work correctly, but once i call them over HTTP request they are not executed. I asked around and i was told to execute them asynchronously outside the request response cycle using celery and a messaging queue server, it worked well, but still I don't understand why i have to execute some tasks asynchronously even when i don't have race condition and there's only one client calling the web service. This is a big black spot for me because I make it work without really knowing how. Can anyone explain it to me?When developing my web application using Django