Why use a different analyzer for "index" and "query"?

[Steven White]

Hi everyone,

In Solr's schema, I have come across field types that use a different logic
for "index" than for "query".  To be clear, I"m talking about this block:

    <fieldType name="text_en" class="solr.TextField"
positionIncrementGap="100">
      <analyzer type="index">
   <!-- what you see in this block doesn't have to be the same as what you
see inside "query" block -->
      </analyzer>
      <analyzer type="query">
   <!-- what you see in this block doesn't have to be the same as what you
see inside "index" block -->
      </analyzer>
    </fieldType>

Why would one want to not use the same logic for both and simply use:

    <fieldType name="text_en" class="solr.TextField"
positionIncrementGap="100">
      <analyzer>
   <!-- same logic to be used by for "index" and "query" -->
      </analyzer>
    </fieldType>

What are real word use cases to use a different analyzer for index and
query?

Thanks,

Steve