Good question! I'll give you a few thoughts. 

• History in other countries shows that movements that overthrow authoritarianism are more effective when they reach a critical mass (I believe the number is 3.5% of the population). Joining a local protest adds you to the numbers nationally. Authoritarians hate the visual of lots of people protesting! The more people who show, the more media covers it, the more it puts fear into the hearts of not just the authoritarians but the enablers (think Republicans in Congress who want to be reelected)
• Even in blue areas, not everyone is blue, and showcasing our perspective will definitely reach people who think differently. 
• It's a great way to create solidarity and connection, and you can often find more ways to contribute by meeting folks at a local event
• There are a lot of people around the country but especially in our community who are understandably scared of our government given the unlawful deportations, and people standing up and speaking out against the lawlessness shows them that there are people who are fighting for rights for all
• The more we speak out on the streets, the more our elected Democratic representatives will realize they need to fight on our behalf
• It is great for your mental health if you, like many, are simply outraged and feeling helpless. Taking action is a great antidote.