I don't understand placeholders in Keynote

I've been happily using Keynote for a few years now, but realize that I'm not sure of the value of placeholders. Not sure what I'm missing.

I copy paste to make new presentations from previous formats. What are the advantages if I were to make new themes from these often-used setups? Would it make my work go faster, or?

Thank you for any info. Still a beginner, alas. 😉