Meaning of prefix по- in verbs поехать and пойти

Question

Both this website and my grammar workbook state that the meaning of по- in the verbs пойти and поехать is to indicate the beginning (but not necessarily the completion) of an action.

However, I thought that they were just (empty) perfectivizing prefixes, to the extent that поехать and пойти can even be used as the perfectives for ездить and ходить respectively.

Question: So if one uses поехать or пойти, can one infer that the action has been (will be) completed, or just that the action began (will begin), without knowing anything about the final results/consequences of that action?

As an example, if I say: Она пошла на работу do I mean, (1) she left for work (began the process of going to work) or (2) she left for and arrived at work, i.e. completed her journey to work?

I.e., does по- here mean (1) the action was begun or (2) the action was completed?

Answer 1

With motional verbs, prefix по means beginning of motion. Inversal case would have the prefix при, when ending the motion (arriving to some place).

Answer 2

In your example,

Она пошла на работу.

It means (1) the action was begun.

It can mean 'She just left for work' or 'She started a new job / resumed her job' [after a break].

You can easily imagine a situation where

Она пошла на работу, но попала в пробку и вернулась домой.

'She left for work, but got stuck in traffic and came back home.'