There are several types and brands available. The major use of acrylic admixtures has been in swimming pools, as a method of bonding portland cement mortar to concrete. Acrylics have been also used for bonding cement plaster to cast in place concrete, and other substrates that are normally too slick, too porous, or not porous enough for a permanent bond.

I started using acrylics in the finish coat in searching for a method to retain the color in mix. Pre-mixed color stucco finishes from California have a small quantity of plastic chemical to retain the color in the mix.

Using acrylics with color not only improved color retention, but I discovered our jobs had no cracks, or at least very few. There also were no check cracks or imperfections from shrinkage, because of the slower final set of the mortar.

Other benefits of acrylics: Increased bonding strength (of course, they are bonding admixtures):

Improved compression strength.

Few to no cracks due to flexibility.

No need to wet the wall before applying the finish coat, which causes loss of
color.

Reduced porousness, reducing water infiltration, and spalling.

Reduced likelihood of efflorescence.

The increased material cost is about 50 cents a square foot, but the the overall quality of the finish product is vastly superior, so I have no problem charging more for my jobs.