The antenna based complex gains vary across the passband, primarily due to the antenna based band-shape and residual fixed delay errors. These variations in the complex gains must be corrected by 'bandpass calibration' before the visibilities from individual frequency channels are averaged. Also before averaging, the bandpass shape can be viewed by the AIPS task POSSM and badly RFI affected channels were edited out which can degrade the quality of final image.
The more channel averaging is done, the more bandwidth smearing will affect the sources in the outer parts of the primary beam. At low frequencies there are always strong sources in the outer parts of the primary beam. The advantage of averaging is, it reduces the data volume and the later processing steps can be accomplished in a faster way and also averaging helps to increase signal-to-noise (snr) ratio.
The situation at 235 & 610 MHz are different because of different interference environment, problem of bandwidth smearing, wide field effects etc. and so they will be treated separately.