A semantic layer is something we use every day. We build dashboards with yearly and monthly aggregations. We design dimensions for drilling down reports by region, product, or whatever metrics we are interested in. What has changed is that we no longer use a singular business intelligence tool; different teams use different visualizations (BI, notebooks, and embedded analytics). 

Instead of re-creating siloed metrics in each app, we want to define them once, open in a version-controlled way and sync them into each visualization tool. That’s what the semantic layer does, primarily defined as YAML. Additionally, the semantic layer adds powerful layers such as APIs, caching, access control, data modeling, and metrics layer.