I find the big benefit of Altair is that the API is so nice and composable, and because vega provides a ton out of the box to build really high quality visualisations.

For instance, if you take this (sample) report using Altair to plot Default Alive / Default Dead: https://datapane.com/leo/reports/startup_finance_report/ - the interactive code is actually relatively small: https://gist.github.com/lanthias/5a41c1e4b21ae274ddb95cf5ad1...

It's also great being able to add Altair shapes to Folium for geoplotting (as the vega geoplotting is a bit more low-level).

What I really think is missing in the ecosystem is a "vega for tables", so you could be rich, interactive tables with a similar grammar. That would rock.