Pros
Cheap Mod
Nagant Mod has a lot of qualities a Mod doll can wish: cheap to raise (as she has viability pre-Mod), cheap to Mod (as she is a 2 to 4 star Mod), tiles and skills that make her worth using, and, lastly, her peak performance arrives at only Mod II.
0s ICD Skill
While it is not explicitly stated, her Skill 2 actually activates with her opening shot. That means she provides a 10% Damage buff at the start of the battle. While the buff is not big, it nonetheless means a buff in early damage only a select few HGs can provide.
Very strong tiles
Nagant Mod’s tiles pack 36% Damage and 20% Crit Rate, some of the best tiles a HG can offer.
Repeating buffs
Nagant Mod's Skill 2 provides Damage buffs for her allies more frequently than the typical buffer HG cooldown of 12 seconds, helping activate skills that depend on buff frequency (e.g. M500 Mod, AS Val Mod).
Night debuffer
Reducing enemy damage by such a large amount, especially at night, can make it so that preventing damage through HP Shields and Armor tanking is more easily achievable. Some easy examples here would be Armor tanking Gunners and using Shields to negate Doppelsoldner volleys.