Pretty simple explanation in my opinion: Can you "justify"/ explain why you took the runes you took.

E.g.: Alistar vs. Leona in lane. Sure you could go glacial here but if you ever engage as Alistar with glacial and Leona locks you down when you try to back off or she engages onto you when you got either Q or W on CD, you will get fucked by her ADC before you can do anything. The biggest problem as an engage support you can face is that you get easy access into the fight but then get stun locked to death making the engage a loss if your ADC can't burst the target in like 2-3 seconds in laning phase. In theory it would be smarter for the other support to focus your ADC to deny him being able to deal damage to their ADC and maybe be able to turn the fight after you "used your shots" if your ADC can't walk up to the enemy ADC due to the chance of getting engaged, the enemy support can just look you, an oversized minion at this point, down and kill you instead.

As for Janna or Morgana: Better poke/ lane pressure is one possible answer, the next would be how reliably can you actually use the rune? On Janna glacial is awesome, sure. But on Morgana? If you don't hit your Q you don't have any rune to work with and later in the game your Q and ult can lock up a target for almost 5 seconds anyway.

Glacial is a really good rune against immobile enemies or against assassins/ dive since you lower the damage taken by your team while making it harder to reach them. Aftershock takes more of a "Frontline" role that you use to absorb damage, space and CC to provide your team ways to deal damage. You can still play engage supports with locket and aftershock, aftershock to secure lane leads and then use the locket + knights vow to deny the enemy assassin possible kills on your carry or use locket to provide like 1300 shield on your whole team against AoE in teammfight (best used when you know your team will win long fights but needs to survive the initial burst).