The term Pack Map describes a composite image which contains multiple single-sample (i.e. grayscale) texture maps, each of which is isolated to a unique color channel. (i.e. Red, Green, Blue or the Alpha channel.)
A physically-based material which neglects to leverage a Pack Map (opting for multiple image files, instead) will unnecessarily bloat the memory overhead of the material, as much as 400% in many cases, even more if additional maps are in-play (e.g. detail maps, alpha masks, emissive maps, etc.)
Packing multiple texture maps in a single image file is not a new practice by any means, but it didn't become a crucial step-in-the-process of Unity-based art pipelines until physically based rendering became the standard. This is in large part due to the additional surface maps required for PBR materials. There is little room for negotiation on this matter -- if your project isn't leveraging Pack Maps to the fullest extent possible, you're leaving performance gains on the table; both in terms of project and build size.