I had internships after junior, senior, and 1st year grad (went straight to grad school for an MS), and even still I wish I had tried for one after my sophomore year. It's a time when you can try out different companies (do you want to work in a large, medium, small, start-up, what kind of company culture, etc.) start making connections within your field (for future jobs, recommendations), gain practical experience, and lots more. They're valuable.