Spinning it keeps it wide open and keeps a steady momentum so you can be ready to throw at any moment. Because you're typically working with a moving target, you don't always know, in advance, exactly when you're going to throw it, and you need to be ready as you're "closing in" on whatever you're trying to lasso.
NuAngel t1_j2257gn wrote
Reply to ELI5 What is the purpose of continuously spinning a lasso before throwing it? by Asian_1nvasion
Spinning it keeps it wide open and keeps a steady momentum so you can be ready to throw at any moment. Because you're typically working with a moving target, you don't always know, in advance, exactly when you're going to throw it, and you need to be ready as you're "closing in" on whatever you're trying to lasso.