This pattern is used when the speaker assumes or acknowledges a certain situation and then states what will happen or what one will do under that condition. It often implies a sense of obligation, determination, or a natural consequence. The nuance is slightly formal and literary, so it's mostly found in written language or formal speech.

The condition is presented as a given fact, not a hypothetical. For example, if you say '先生の頼みとあれば', you're saying 'if it's a request from the teacher', treating it as a known circumstance. This pattern is not used for general 'if' conditions like 'if it rains'; it's for when the situation is specific and often carries an emotional or logical weight.

Because it's N1 level, it's important to use it in appropriate contexts. You might hear it in speeches, news, or formal writing. In casual conversation, it would sound too stiff.