What is purpose of frequency sweep and center frequency?

I am having a hard time understanding why in HFSS we set a frequency sweep and center frequency? Why not just center frequency?

Also, when checking S11 results, do I check it only for center frequency or look at the whole frequency sweep range?

Thanks.

No matter talking about antennas or RF/MW devices, we are always interested in their impedance bandwith. Therefore we need to observe their Z-parameter or S-parameter in a specified frequency range, what's why a frequency sweep is required. As for the center frequency, it is set for mesh generation (assuming the center frequency u mentioned means the solution frequency in HFSS).