A Tier I account is the primary, default account you open when you enrol in the NPS, and it's the one that comes with tax benefits, including deductions under Section 80C and an additional exclusive NPS deduction. In exchange for these tax advantages, this account comes with restrictions — your money is largely locked in until you reach retirement age, with only limited, conditional partial withdrawals allowed before then.
Every NPS subscriber must have a Tier I account; it's not optional, and it forms the core of your NPS-based retirement planning. The lock-in exists specifically to ensure the money is used for its intended purpose — building a genuine retirement corpus — rather than being withdrawn and spent for other needs along the way, which is precisely why it comes paired with more generous tax benefits than the optional Tier II account.