Altair: Declarative Visualization in Python 8 years ago
Altair enables you to do very quick data visualizations of your data - this is crucial when doing Exploratory Data Analysis. There is minimal code to write to get sophisticated plots. That to me is its strength.
Bokeh's strength (for me) is when I need to heavily customize a visual to an extreme. I know of some who use it for streaming data - etc which cannot be done in altair.
Additionally you would only want to use Altair on medium sized data. But Bokeh in combination with Holoviews and Datashader can render incredible amounts of data (see pyviz.org)