For starters, without a means to give them haste, you have to wait a turn to see any benefit from a mana dork outside of chump blocking, which you arent going to want to do in the early game or in the face of creatures with trample
If you do, add three mana in any combination of its colors and put it into your hand
River Boa (at that time) was considered "very good", with two abilities (Islandwalk and Regeneration) and a 2/1 power/toughness at a casting cost of only two mana
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Whatever reason you might have, it can be okay to go into a draft planning to go for a certain color, combination, or archetype
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