Is flux necessary for soldering?

I'm curious, could you please elaborate on the role of flux in the soldering process? I've heard that it's crucial, but I'm not quite sure why. Could you explain how flux assists in the joining of metals during soldering? Also, are there any specific types of flux that are more suitable for certain types of soldering tasks? I'd like to have a better understanding of this aspect of the soldering process. Thank you for your time and expertise in this matter.