Installing stone veneer - weather barrier necessary?

I'm installing a stone veneer on top of a brick/cinderblock chimney with 2x4" studs and 7/16" OSB on top of that. The website for the company says to use 15 lb asphalt-sturated rag felt or ASTM226 over the OSB. This is for an inside fireplace. Is the weatherbarrier really necessary? I couldn't find a distinction about whether they were referring to an indoor/outdoor installation.
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