Taking load off the Master server by using a slave
Yes.  A slave can (and should) offload SELECTs from the Master.

If you outgrow one slave, you can have many.  This gives you unlimited read scalability.

Keep in mind that every write that happens on the Master must be replayed on every Slave.  Using Row Based Replication helps make this less of a burden on the Slaves.  But, eventually, you may become write bound.  (That's a separate topic; at 20/sec, you are probably nowhere near that.)

A "critical read" is one that depends on something that was just written.  Regular replication is "asynchronous", hence a write can take arbitrarily long to get to the Slave.  Hence, if, say, the user posts a comment, he will want to see it immediately.  The post must happen on the Master.  The "see" is a SELECT that would be nice to do on the Slave, but cannot be safely done ("critical read").  Other reads -- sure, do them on the Slave.