When using composite models in Power BI, you have a choice for how to store your tables. Storage mode gives you options, but why would I pick one versus the other for my dimension tables? Adam has you covered!

Use composite models in Power BI Desktop
https://docs.microsoft.com/power-bi/transform-model/desktop-composite-models

DirectQuery model guidance in Power BI Desktop
https://docs.microsoft.com/power-bi/guidance/directquery-model-guidance