HPMC (Hydroxypropyl Methylcellulose) is commonly added to dry mix mortar for several reasons:

1. Water retention: HPMC can improve the water retention capacity of dry mix mortar, which can enhance workability and prevent the mortar from drying out too quickly.

2. Improved workability: HPMC can improve the workability of dry mix mortar by providing better spreadability, leveling, and smoothing.

3. Improved adhesion: HPMC can enhance the adhesion of dry mix mortar to the substrate, preventing it from peeling off or cracking.

4. Reduced shrinkage: HPMC can reduce the shrinkage of dry mix mortar during the drying and curing process, which can help to prevent cracks from forming.

5. Better resistance to sagging: HPMC can improve the sag resistance of dry mix mortar, making it suitable for vertical applications.

6. Increased open time: HPMC can extend the open time of dry mix mortar, allowing more time for the mortar to be applied and adjusted before it sets.

7. Improved durability: HPMC can improve the durability of dry mix mortar by enhancing its resistance to water, freeze-thaw cycles, and chemical exposure.

In summary, HPMC is added to dry mix mortar to improve its water retention, workability, adhesion, shrinkage, sag resistance, open time, and durability, making it more suitable for various construction applications.