Tomato paste can taste rather blunt when stirred straight into a finished dish. A short spell in the pan with the softened onion and oil helps it become part of the base.
Add the paste over moderate heat and stir so it does not catch in one spot. It should darken slightly and smell fuller, not burn black. Then add tomatoes, stock, or water and scrape up the flavorful coating with a suitable utensil.
Because paste is concentrated, a small amount can change the sauce considerably. Add according to the recipe or in modest stages, especially in a pale bean dish where you want only background depth.
If the pan is dry, add a little liquid sooner. The goal is cooked tomato flavor, not a demonstration of how long you can leave a thick ingredient against hot metal.