The existence of the Rescue armor is an indication that final fight against Thanos and his minions. But even before that, the movie already revealed its headpiece, hinting that sooner or later, it would be shown in action.
As with all variations of the Iron Man armor in the MCU, the Rescue suit was also created by Tony Stark. In Endgame, it was revealed that it's an anniversary gift to his wife that somehow, Morgan, found lying around in their garage. This was during the five-year gap between Thanos' decimation and the time heist which the genius, billionaire spent living a quiet life with his wife and daughter. Tony jokingly complained that Pepper never wore the armor despite it being a present, but considering this period in the MCU, there's really no reason for her to don it. In case anything happened, her husband can simply suit back up as Iron Man to protect their family.
In hindsight, this makes Tony's decision to Tony was confronted with his mortality. That, once again, happened at the beginning of Endgame, when he didn't think that there's any way for him to come back home after his trip to Titan.
Even when Thanos won in Morgan Stark. His daughter was far too young to understand this, but gifting his wife a powerful suit gave him reassurance that in the event that something happened to him, she has the tools to protect their family. If anything, perhaps Tony has gotten more paralyzed about the idea of anything happening to them, especially as he finally experienced what it was to have the normal family life he longed to have.
Overall, despite Tony's initial skepticism to the time heist, he, somehow, always knew that there would be a time when he'd have to suit back up, regardless if that meant having to face Thanos again or any other threat. Pepper knew this too; she itted that convincing Iron Man to fully retire was one of the few failures of her life. Maybe, she even had an idea of why her husband created the Rescue suit in the first place, and she's been secretly learning how to operate it. Otherwise, how would she be able to easily maneuver it during the final battle in Endgame?
