How important is a "list" to paticipate in the launchs?

When reading different posts in forums of JV notify Pro, it seems to be a certain strong focus on lists.

So far I haven't done any listbuilding, and I'm wondering hos important it is to have list to participate in the launches?

It would be absolutely possible to set up a site for product that launches during the next days/weeks, but I then have to get my site, getting a lot of backlinks and so on?

If such a email-list is critical, how do I use Scrapebox acchieve this goal?