Exercise 2: Read Replicas vs. Multi-AZ/HA Standby Replicas — Possible Solution
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What each is actually for:

A READ REPLICA exists to SCALE READS -- it's a read-only copy of the
database that application traffic can be directed to for read queries
specifically, taking load off the primary instance so it can focus on
handling writes. Its purpose is performance/throughput under normal,
healthy operation.

A MULTI-AZ/HA STANDBY REPLICA exists for FAILOVER -- it's a copy kept
ready specifically so that if the primary instance fails, the system
can automatically promote the standby to become the new primary with
minimal downtime. Its purpose is availability/resilience during a
failure, not day-to-day performance.

Can one serve the other's purpose by default?

Per the chapter, NOT by default -- these are described as distinct
purposes despite the underlying replication mechanism often being
similar. A standard HA standby typically isn't meant to serve regular
application read traffic during normal operation (it exists purely as
a failover target, often not even accepting read queries directly). A
standard read replica typically isn't automatically promoted to
primary the way an HA standby is if the primary fails -- promoting a
read replica to take over as the new primary is often a manual or at
least separately-configured process, not the automatic behavior built
into HA/Multi-AZ setups specifically.

The chapter does note that some provider configurations blend both
roles into one replica that serves reads AND can be promoted on
failure -- but this is a specific configuration choice, not the
default behavior of either type on its own.

WHY THIS WORKS AS AN ANSWER
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This distinguishes the two by their PURPOSE (scaling vs. failover)
rather than just their mechanism (both use replication), which is
exactly the distinction the chapter calls "genuinely easy to confuse"
-- and directly answers the "can one do both" question with the
chapter's own qualified answer: not by default, though blended
configurations exist as a deliberate exception.