see e.g. https://youtu.be/iHSobh2rtyg?t=6m (6:00 -- 6:06)

You've created a blueprint that you want to build many times in the early game. The blueprint contains 4-5 entity types but they are often rotated and/or positioned in a way that makes it hard to place the them quickly.

Using the blueprint saves you some mental effort when you want to reuse this ...