Likes are useful for small-medium sized accounts that don't receive a lot of engagement.

Posting a lot of small talk is not everyone's cup of tea, plus it clutters a feed with noise. Likes is a more efficient way of showing non-verbal agreement.

Artists posting art can get a general idea of how their followers like different types of artwork by looking at the differential in likes. It's a feedback mechanism.

Small-medium accounts may not receive much engagement so likes are a useful indicator in finding others of similar disposition. Seeing the same name/avatar repeatedly helps connect names with people.