Sometimes you're limited in space and don't have the opportunity to take the larger width. Or in RF-applications, where an increased metal width has worse properties compared to narrow trace width. E.g. consider an on-chip transformer, you don't want to use wider width than necessary because of trace parasitics. And it becomes even more important with increasing frequency.In the case of dc-current or low frequency application, I agree with you, let's say for supply routing and if you have the space!