Why do you use polymer-modified mortars for repairs?

Polymer-modified mortars contain resins (such as acrylics or SBR) that significantly improve the performance of the cement. Unlike standard sand-and-cement mixes, polymer mortars have excellent adhesion to existing concrete, reduced shrinkage (meaning fewer cracks as they dry), and lower permeability (better waterproofing). They also offer higher tensile and flexural strength, allowing them to withstand the movement and stresses of the repaired structure much better than traditional concrete mixes.