EIN stands for Employer Identification Number. It's essentially your business's social security number. You use it to identify the business.
According to the IRS website, you need to have an EIN if you operate as a corporation or if you have employees.
I say always. Getting your EIN is super easy. It takes about 5-10 minutes, and it's free. You will need an EIN to open your bank account, and as a sole proprietor or single member LLC owner, you can use your EIN in place of your social security for important documents like W-9s. That means, in short, that you don't need to give your social to strangers.
Don't pay someone to get your EIN for you unless it's bundled into a full service package. At Braden Drake Law, I do the EIN application for clients but if you came to me and just needed an EIN, I'd tell you to DIY it because it's super easy. Some online legal sites charge up to $80 to do your EIN for you, which is bananas. That's why I made a step-by-step tutorial video to show you how to do it on your own for no cost. Check out the box below to watch that.