Where and when I can use FFT object ? please teach me!!

Hello ,everyone

I am deeply stuck with FFT filter now.
All I know that is transforming frequency domain to time domain or the other way around.

Currently ,my mind is SO what ??? I can see two side of signal.So what ??? Anyway That's same things talking difference ways. so what ?? etc.

I would like to know what is function of FFT filter in synthesizer ? and when , where I need ? does it change the shape of sound ? I think it's not.I think we can observe two difference way of sound spectrum .
I am starting building patch and learn object.
In FFT part It's hard to catch the point to using FFT filter.
Please teach me to understand the point of using FFT and change my perception to look at a little difference way so I can learn and improve skill.

many thanks.

lu