A trip through Asia usually crosses several mobile networks, and each border is a decision: pay whatever your home operator charges to roam, or buy a new local SIM. A regional eSIM removes both. One plan stays installed as you move between Afganistan, Ermenistan, Azerbaycan, Bahreyn, Bangladeş ve Butan and the rest of the region, and it keeps working without you touching your phone at a crossing.
The rule of thumb is simple: two countries or fewer, buy per country; three or more, a regional plan is almost always cheaper and always less work. Single-country plans usually give you more data per unit of spend, so if one destination dominates the itinerary it can be worth buying that one separately and treating the regional plan as backup.
Asia spans the fastest mobile networks in the world and some of the most restrictive, so coverage varies more here than on any other continent — the plan that works in Seoul is not the plan that works in Kathmandu.