Something they’ve noted about this debate that will different from other debates:
The age of Trump and Biden (74 and 77, respectively) means a significant chance of the VP becoming president at some point in the next 4 years.
If Biden were to win, he might not run for re-election, which means Harris would be a likely frontrunner in 2024.
People are interested to see how this debate will progress after the last presidential debate was so chaotic. Pence and Harris will likely be able to dig deeper into content than the presidential debate.