As per MGMAT SC guide, Subordinators such as 'BECAUSE' create subordinate clauses, which can in turn attach to a main clause with a comma. For Example:

I need to relax, BECAUSE I have so many things to do.

Is the comma optional in this sentence? I know that the comma is optional for Coordinating Conjunctions such as AND that can connect two main clauses. Is that true for Subordinators as well?

If not then why is the following usage listed as right in the MGMAT SC guide:

Plants grow BECAUSE the sun shines.

Both the examples I cited are from MGMAT SC guide. I searched in the forum but couldn't find an answer. Can anybody shed some light on this issue? Thanks in advance.