Q&A: How and why AP called Clinton-Sanders race when it did

News organizations widely rely on The Associated Press to call the winner in elections and, in presidential caucuses and primaries, to determine the number of delegates won by candidates to the nominating conventions.

[...] entrance or exit polls of voters arriving to caucuses or leaving voting precincts (on Monday, entrance polls were used).

[...] an analysis of actual votes from a random sample of precincts across each state.

[...] the AP vote count, which tabulates all actual votes as they are reported from precincts to counties, or towns or parishes.

For the very closest races, the AP depends heavily on actual vote returns, as they slowly or rapidly flow in depending on circumstances, to make a call.

All the results had been tallied, the Iowa Democratic Party told the AP it would not conduct a recount of any results, and the Sanders campaign said it was not interested in challenging any results.

Iowa Democrats award delegates proportionally, based on the statewide vote and the vote in individual congressional districts.

Q:

Since delegates determine who wins the nomination, and Sanders and Clinton split them almost equally, why does it matter who got the most votes in Iowa?