The video makes it fairly self-explanatory, tbh (it’s a minute), and I only watched ~15 seconds with the sound off. If you go to invite someone to your guild through the Guild Admin > Recruit > Invite button, you enter their invite code and their player profile pops up, allowing you to press Recruit – and also ‘Fight’, I’m assuming.
How you hope your defence performs, and how it actually does, based on the moves chosen by the AI, is useful knowledge, and I don’t think I’m the only one who would be familiar with players in Guild or Global chat asking for others to test their defence.
Watching someone else play against your defence might help someone realise that choosing The Gray King isn’t as effective as they might have hoped, since it might not choose the right colour gems to destroy.
I’m still in a GW bracket with non-fully-participating Guilds, so we don’t actually get a lot of quantitative feedback on our defences (which sucks). I don’t think it takes that huge of a leap of imagination to consider that someone might want some insight into how their defence performs, and more than just “I beat u haha nub”.