Something that takes 30 seconds to say takes 30 seconds to forget.

Writing ensures that

  a) it was important enough to spend time writing out
  b) the ask has been thought through and can be communicated in clear terms (and via written clarifying questions)
  c) the conversation can scale beyond half duplex in the same space and timezone
  d) there is a log of the ask, questions, responses, etc. for posterity