Mt. Ember is located to the north of One Island. While it is not required, you can go to Mt. Ember at any point in time after defeating the Cinnabar Gym Leader, Blaine and completing a quick side adventure with Bill in the Sevii Islands.
The main reason to go to Mt. Ember before the Elite Four is to capture the Legendary Pokémon, Moltres. However, you need to defeat the members of a bike gang on Three Island before you will have a chance to capture it.
After defeating the Elite 4, you will be asked to find a Ruby Plate for Bill's friend Celio, which is part of a side quest that, when completed, will enable trading between Ruby, Sapphire, Emerald and Colosseum.
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