Purpose of shareUID

Question

I understand that two things, when two applications shared the same user ID:
1. they can access each other's data and, if desired, run in the same process.
2. they are considered as one identity.
Thus if app 1 and app2 each hold a specific subset of ~different~ permissions, during a permission check in the OS, each app inherits all the permissions from the other.

I'm trying to understand the advantages of these assumptions listed above? For example,
1. Advantages because the operating system saves system resources by holding to apps in one process instead of two? Easier means of communication between the apps, as well as easy access to each apps' data? Are these correct, if so are there any other reasons?
2. If the assumption is correct, why is it such a way? Why not give each app a different uid without the option of sharing a uid between apps?