Chevy Auxillery Battery

For the last 15 years, the truck was used for towing a TT. When not towing, the truck sat a majority of the time. Did not originally replace the 2nd battery until the starting battery died 7 years ago. Replaced both then. Now the starting battery has died and I have replaced both again. Moved from a TT to a MH last fall. Now truck is going to be used for work commute. Have always been curious about the need of the 2nd battery since we never used a TC.
Thanks for the insight, and sorry for not mentioning Gas or Diesel to start with.