Because a dam was the most useful thing a ruler could give — it turned uncertain rainfall into a dependable harvest — and because recording it in stone was how a ruler claimed the credit permanently.
1. Irrigation was the foundation of the whole economy. The chapter states that agriculture was closely dependent on irrigation, that considerable importance was placed on the construction and maintenance of reservoirs, canals and dams, and that facilitation of agriculture remained a major preoccupation of the state because it was one of the major sources of revenue. With the basic land tax at generally one-sixth of the produce, more water meant more produce, which meant more revenue. A dam paid for itself.
2. It affected everybody, and lasted. Irrigation structures helped store and distribute water, supported agriculture and encouraged the growth of settlements. Unlike a battle, whose result fades in a generation, a working tank feeds the same fields for centuries.
3. Recording it served the ruler. An inscription is a public, permanent statement. Rudradaman I's long Sanskrit inscription on the Junagadh rock describes his military achievements and the repair of the Sudarshana Lake side by side — welfare works were treated as being just as much a proof of a great ruler as victories. That fits the chapter's account of kingship: the king's first duty was protecting his subjects and doing good to the people.
4. Recording it also served the administration. A dated record of who built or repaired a structure, and under whose orders, is an administrative document — evidence for later officials of what exists, who is responsible, and when it was last repaired. This is why the Junagadh rock ended up with three records of the same lake by three dynasties across about 700 years: Puṣhyagupta's original dam under Chandragupta Maurya, Rudradaman I's repair (c. 150 CE) and Skandagupta's restoration (5th century CE).