So these circumstances are probably a bit niche, but I think a lot of D&D is getting into niche situations hahaha.
1. You are in a fight with an enemy, the enemy has made you rattled somehow but is staying in melee. You decide to use your action to make a persuasion check to talk the guy down, as long as it is an honest plea you now are no longer rattled until the end of your turn. That means your persuasion can benefit from any expertise die you might have. You also do not have disadvantage for fighting the guy if the check uses Int, Wis, or Cha. Continuing this situation, he doesn't go for it (after all you might not have disadvantage but the DC might be very high) so you decide to move away from him. This provokes an opportunity attack, he hits and deals a lot of damage, but since you are currently immune to rattled you are able to use Force of Will as a reaction, much to this enemies surprise.
2. Lets say you have acquired feather fall from any number of sources. The enemy has rattled you, they also have a hostage on a cliff they are threatening to drop if you don't toss your weapons away. They know they have cast a spell to rattle you, they see you having trouble getting your bearings, so they are only giving you this turn. You use your action to once again make a persuasion check, or maybe you heal an ally. Suddenly you have managed to focus on the need of the person in danger despite whatever the caster did to you. He sees you are not surrendering and as a readied action tosses the hostage off the cliff. Since you are not rattled until the end of your turn you can cast feather fall on them as your drive to help them overcomes the evil magic.
3. You thought you had the mad warlock cornered. Little did you know he had an escape plan. His former base, now in ruins and torched, is all that remains of the battle you just had. You need clues, time to do some investigating, but everything is charred so you would normally have disadvantage or penalties to this. However, since you are such a devout follower of your entity you do not have these problems (as long as it is Intelligence based) for they gently guide your investigation.
Something to remember: being immune to rattled allows you to have a reaction, as well as gain expertise die. So if you have some nice expertise in skills but you have become rattled (possibly for a long time) you can still benefit from those as long as you are being honest and non-violent.
I think the huge part is no disadvantage or penalties on Int, Wis, and Cha checks. That will usually cover talking to people (even enemies), seeing through lies, finding hidden things, and knowing obscure knowledge. No disadvantage for being in combat, obscure knowledge, lightly obscured areas, being poisoned (unless the narrator calls it a Con based check), or any number of things that cause disadvantage.
It also completely negates the first level of Strife.
