The original meaning of a trigger is "to pull a trigger of a gun" and in the measurement field, it means the function that sets the timing for fetching waveforms to enable observation of apparent periodic signal waveforms by stopping with an oscilloscope.
When a trigger is not set in the FFT analyzer (trigger free), problems occur whenever data is fetched repeatedly such as  periodic waveform does not seem to stop on the time window of FFT and  sporadic signals cannot be successfully fetched in the time window of FFT. A trigger is used to prevent such problems.
The FFT analyzer is essential for the following
The FFT analyzer is essential for the following purposes in addition to the reliable monitoring of temporal waveforms.
- Synchronous addition of time axis waveforms and Fourier spectrum