A simple explanation to your question is the a logical abstraction.
What is faster?

### 1. Parallel Execution

1. Analyse query
2. Determine which index to use
3. Split the query into parallel reads
4. Consolidate query results
5. Present consolidated data

### 2. Serial Execution

1. Analyse query
2. Start retrieving data (serial)
3. Present data

## Your Queries

A simple `SELECT <column> FROM <table>;` will be faster and the most cost-effective with a serial execution. It's already buffering data to the client before the second option has even started splitting the query into parallel threads/tasks.

However, a `SELECT MAX(<column>) FROM <table>;` will benefit from parallel execution, because each thread will be able to retrieve the `MAX(<column>)` value from a portion of the table and then pass that on to the client.

BTW, what were the execution times for the `SELECT MAX() ...` statement?