Channel choice is usually made because "a competitor was there" or "the agency suggested it". But the right channel depends on where the audience is and what the message is. If the ICP isn't written down, channel choice is random and budget leaks.
Positioning comes before channel
Until you've clarified who you sell to, on which problem, and against which alternative, no channel runs efficiently. Once ICP and positioning are in writing, the channel mix becomes a result, not a guess.
Being written means being testable
A written ICP lets the team aim at a common target and keeps the message consistent across channels. It becomes testable, improvable and transferable.