Generally speaking, having more votes than chairs promotes consensus candidates at the expense of controversial candidates.

Basically, after you select your dream council of 20,you can chip in a few more votes to candidates that compete with someone you would hate to see elected.

However, Hive witness voting is unusual because the ranking be low Top 20 matters too, so this might have been the primary motivation. You do not just select the Top 20, you also ship some money to backup witnesses by issuing tickets for the 21st witness lottery.