Second Main Phase and Mana Drain

[Turtle]

I have heard mention of announcing that you are moving into your second main phase when playing with Mana Drains.  I am looking to gain a better understanding of why you would want to do this and what advantages it affords. Is it to ensure that if your opponent tries to disrupt you on your turn, and you Mana Drain their spell, that you receive the mana on your next turn when all mana sources have untapped as opposed to your second main phase? Are there any other reasons? Thanks for your time!

[Limbo]

The reason you state is indeed the main reason. A second one is that you can try to mind-trick your opponent into having a mana drain at the ready (like inspecting your graveyard to fake the presence of a yawgmoth's will in your hand).