If a red mana you add has certain restrictions or riders associated with it (for example, if it was produced by the last ability of Arena of Glory), they'll apply to that mana no matter when you spend it
Mardu Surge Strike While The Iron Is Hot Ally with the Mardu clan to make attacker tokens and sacrifice them for value with this Red-White-Black Commander deck
You see, whilst the Horizon lands are definitely going to see more play and have much more raw power to them (theres a reason why even monocoloured decks like Hardened Scales still wanted to play Horizon Canopy), they largely just improve previously existing strategies, helping ensure those decks flood less and can have more impactful topdecks as the game goes long
however, they are not necessarily rare because they are so expensive - many of the rarest cards were given away for free, or could be pulled from packs
For just two mana, she provides a power boost for all of your Dwarves, free Treasure whenever any of them become tapped, and, most notably, an outlet that lets you convert Treasures in play to threats from your deck