Protests (especially when held on a weekend) are a low-investment way for people to dip their toes into civic engagement outside of the voting booth. They also tend to be fun and positive, which is what you want for newbies
No kings has gotten so many of my family members who have never done, like, anything to show up. Do I want my family to do even more? Absolutely. But this got them to take a first step, you know?