What is the reason we read post hoc? I know we read it when F is significant, but why?

2ndly, why r these 2 output readings different?

1) Post hoc tests reveal a sig difference between Level 1 and Level 2 (mean difference=12345, p=123)

2) Post hoc tests reveal a sig difference between Level 1 and Level 2 (p=123)

When do we insert the "mean difference" into the output?

Thanx in advance