Hey there and welcome!

I personally see the Bash event as a means to try out the latest multiplayer levels with a relatively large amount of players, something that's hard to setup outside of planned events. Competitive multiplayer usually sticks to a limited selection of levels. It's also nice to chat and relax with 'retired' players that may decide to join for the occasion.

Slightly outdated info on the Bash
The level packs that were hosted during past iterations
The 20th anniversary web portal has some Bash info as well

Also, the latter Bashes have some gameplay recorded on YouTube. That should give an impression of the chaos gameplay to be expected. Here's some footage from this years Bash.