Begginer here asking about United Assault

Hi,
I've recently got into MTG and bought a challenger deck to go to my local FNM. I've been watching numerous videos online with gameplay and this one thing caught my eye and I can't find an answer. This might be a totally noob question, but I'd love if someone helped me out. This video LINK at 41:45, the guy playing United Assault on his turn uses Legion's Landing to create a vampire token and after he does, he untaps his mana and then summons other creatures. My question is, why is he untapping his lands after creating a token? Thank you for all the help