The most common reason people port is dissatisfaction with claim settlement experience, premium hikes that outpace what competitors charge for similar cover, or simply finding a policy with better features (higher sum insured, wider hospital network, additional benefits) at a comparable price.
Portability isn't automatic or guaranteed — the new insurer can evaluate your application based on your current health status and may apply fresh underwriting terms, though they're required to honour the waiting periods you've already completed on your existing policy for the same sum insured. You typically need to apply for portability at least a set number of days before your current policy's renewal date, so it requires some advance planning rather than a last-minute switch.
Before porting purely to save on premium, it's worth comparing the actual claim settlement ratio and network hospital list of the new insurer, not just the price — a cheaper premium with a worse claims experience isn't really a good trade during the one moment you actually need the policy to work.