It's possibly the same reason people wear clothing with a large brand-name printed across the back/front. People often associate brands with their "tribe", and liking posts could be another way to signal that they are part of the in-group.
Disclaimer: I possibly don't know what I am talking about as I don't really know Instagram mechanics at all, but I am guessing they are similar to Facebook's.
Disclaimer: I possibly don't know what I am talking about as I don't really know Instagram mechanics at all, but I am guessing they are similar to Facebook's.