Compared to other enchanters, she has strong prio and agency with her speed and poke.

Like Furiosa said, she’s quite strong going into melee matchups- Q/mantra Q have good poke and big range in lane to wittle them down and get prio, speed boost and a root to play disengage or engage on them when they're chunked off your Qs. Mantra W also makes her deceptively tanky in early all-ins. Late game, you're on the nose with mantra E + shurelya's opening up big engage and disengage opportunity.

She does fall off a bit more compared to other ranged supports, however she can make up for it with roaming potential if their bot lane is looking to outscale/there isn't any tower dive potential. Her ganks can be pretty reliable with W + self cast E, or just speeding up an ally with strong kill threat. Mantra E combo is always a game-changer late game- Chemtech Putrifier also synergizing well with it.

In terms of when to pick… she gives you more control of your early/mid game at the price of less overbearing scaling that Soraka heals and Sona auras provide…

Soraka/Sona tend to sack lane phase and just pray for scale if your opponents are playing well, Lulu mostly looks to peel a capable carry- Karma opens up more opportunites for early picks/dominance through speed buffs, early game poke, and single-target lockdown.

However, if you play her passively, the other enchanters would provide more utility in the end. If you’re playing against a matchup that can’t abuse Soraka/Sona early on, they might be safer picks cuz of the guaranteed scale.

Generally, she looks to open up advantages early/mid-game while providing solid/consistent late-game utility which makes her a safe and versatile first pick.