Packing measures teams and players ability to take out opponents with forward passes and dribbles. And it looks as if the more you can do this, the more likely you are to win.
The Packing concept is explained in more detail on this article Concerning Packing and in the Glossary.
Bypassing players through passing and dribbling is clearly advantageous as on completion of the action there will be less opponents between you and the goal. Simple!
The metric we are arrive at to assess Packing Effectiveness is called “Impect”. Don’t blame me, a German sports analysis company invented the concept. Total Impect is the sum of all scores from packing whereby if you take out a defender you get 3 points, a midfielder 2 points and a forward 1 point. Generally, the higher the Impect the more opposition players you have bypassed during the game.
2018/19 Season
If we rank Celtic’s matches this season by lowest Impect score to highest we get the following (wins are highlighted Green, draws Amber and defeats Red):

All Celtic’s non-winning matches are in the bottom half of Total Impect scores. Whilst the top 13 Impect scores from 27 matches have all resulted in wins.
This table also highlights how effective Livingston were in both matches in limiting Celtic’s passing game.
The lowest Impect of the season was in Rosenborg where Celtic were, rarely, playing for the draw they needed.
But overall I think you will agree that there is a strong correlation between Packing effectiveness and game outcome.
2017/18 Season
Performing the same analysis for last season shows a similar picture:



Note that the two wins with low Impect were unusual. Against Hamilton Academnical, the opponent had a player sent off in 42 minutes and sat in very deep after that making Pack Passing difficult. And of course, in the 3-2 win at Ibrox Celtic were down to 10 men for most of the game.
I hope this helps to clarify why the Packing concept is a helpful aid to understanding performance.
Post Script: You may notice that average Total Impect has gone down from 307 in 2017/18 to 269 in 2018/19. Capturing the data takes judgement and this season I have been more rigorous to only count players packed that affecting the play. So Celtic are not getting less effective, the data inputter is getting stricter!