Why add latex to adhesive?

Why did our local tile store sell me a latex additive to mix into the adhesive before I adhere my ceramic tiles to the walls/floor? It's all ceramic tile with a pretty 2" border tile (in netting) made of marble. The adhesive is standard thin-set dry mix mortar... just add water. Thanks for your help!
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snipped-for-privacy@yahoo.com says...

in the long run.
Why did you buy it if you were unsure of the need? The tile guy could probably have explained it.
Have fun with your project. Let us know how it goes.
Marc
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The guy basically convinced me to get it, and it was only maybe $13 so I figured I'd return it if you folks said it was a waste of time and money. I've mixed thin-set mortar plenty of times but never added latex to it. Typically, on the floors, I screw down 3/4" ply, then spread flexbond and lay 1/2" cement backerboard. Then, I just lay tile on regular thin-set. This guy told me to add laytex, especially when I use the same 12x12 tiles on the walls (yes, my wife wants 12x12 on the walls...) even though my wall installation is done by screwing cement backerboard onto the studs then either using ready mixed mastic or, in this case, thinset again. Still think I should mix the latex, or am I just thinking about this crap too much -- just do it? Opinions? Thoughts? thanks!
says...

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