You will learn how much you either love rounding or hate it, and how much that will impact your choice of specialty. Same for the OR, and the clinic. Honestly, if I didn't hate rounding/wards, love the OR, and love clinic (way more patient interaction, way less insanity, compared to wards), I have no clue what I would have picked. Rads, I guess? That was fun.
Oh, on another note, shadow as much as you can before med school in various specialties and then do as many different rotations as you can in med school. If you think you're going IM, don't do all IM subspecialties - you can figure that out during residency. Do as many different things as you can, so you don't miss out on something you never really considered that you turn out to fall in love with. There are tons of docs out there that got into a rotation last minute in M3 and changed their whole career path. Also, don't compare specialties by your role shadowing, or by the resident's role on the team, but by the actual attending's role. This will be hard to imagine as a premed but gets easier in M3. But as an example, don't compare your ophtho experience to your gen surg experience as standing in the corner of a clinic room vs seeing the cool parts of a bunch of surgeries as you run back and forth between the ORs. Compare instead the one time you got to sit behind the slit lamp and see what the attending ophthalmologist sees all day to the day of one surgery doc, including the morning rounds and all the waiting between surgeries. It helps put things in a more realistic perspective.