Why is the first field of a composite index so important

If the non-clustered index is a composite index, the first field is most important. Why is it so important?

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Because statistics are only performed on the first field on a non-clustered index, it means that the selection of the index, and which index is selected, depends on the first field. The system statistics of the database are the basis for executing the plan.

So instead of building a composite index, build a contain column?

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