Creatures you control can't have -1/-1 counters put on them.
Creatures your opponents control lose infect.
Melira is a combo enabler: she stops -1/-1 counters from landing on your creatures, which turns Persist and Undying creatures into infinite loops with a free sacrifice outlet. Ramp into Melira, assemble a Persist creature plus a sac outlet, and either drain the table or generate infinite tokens/mana, while also hosing infect strategies along the way.
Cheap two-mana commander that fits any mono-green shell and ramps out fast
Enables clean, compact infinite combos with Persist creatures and a sac outlet
Total immunity to poison/infect and -1/-1 counter removal effects
Combo pieces are individually low-cost creatures easy to tutor and recur
Mono-green has weak interaction, few counterspells, and limited targeted removal
The combos rely on Melira plus two other pieces—vulnerable to spot removal and disruption
Melira herself is a high-priority kill target that opponents will answer
Outside her niche her ability is narrow and does little against most decks