Galadriel boost Rosie, Rosie boost hobbits, hobbits.... win.
The goal of the (MotK) Rosie Cotton deck is to max out and give a huge boost for an attack or defense in the combat phase. Playing it is very comfortable because it's versatile: it can go on a quest + further boost willpower, or boost attack or defense if needed. She can do quest + willpower + fight if she has enough Fast Hitch.
The association with Galadriel is very nice thanks to Nenya who will boost Rosie very strongly. You can choose depending on the game whether you prefer Nenya's boost for the quest or for the fight. So the deck plays on lots of attachments that boost in . Strider is there because we will have less than 6 characters for a very long time. The secret makes it possible to generate a lot of resources with Resourceful in addition to the Steward of Gondor (which is not obligatory at all because one also the red book to gain resources)
Fatty Bolger saves time setting up the two decks. The rest of the deck is there to quickly draw our cards.
Which forces Tom Cotton in and therefore hobbit allies to take advantage of his ability.
We find ourselves very quickly very low in threat (especially thanks to Merry) which allows us to trigger all the cool effects (Sam Gamgee readying, Pippin drawing, the allies are boosted, we cancel the shadows with Staff of Lebethron, we attack strong with Dagger of Westernesse etc.) There are quite a few hobbit allies to stay thematic and to have the boost of Tom Cotton who will defend very quickly at 6/7 without Rosie's boost. He will be a sentinel thanks to Arwen to go and defend the enemies engaged with Rosie's.