The strength of lime mortar is less than cement mortar, the advantages of adding lime in mortar are as follows:
Shrinkage in mortar is less, thus less liable to cracks due to shrinkage.
Lime increases the workability and plasticity of the mortar mix.
Lime has a good water retention capacity and does not evaporate quickly. Also, dry bricks are not able to suck water from the lime mortar.
Lime increases the volume of mortar and fills the voids thus making it water resistant. So, lime mortar provides more water tightness and resistance against rain penetration.
Bonding of lime mortar with bricks is better.
Cement-lime mortar is more flexible and can accommodate the normal movements of brick masonry without cracking. Thus, cement lime mortar, in general, is less liable to cracking than cement mortar.