She does a little bit of everything because she has no identity. She does minor things other support champions can do.
The only thing she can't do is stun or heal her ally (unless she gets Athene's.)
Q: A very basic projectile spell. It can't pass through enemies and hits in a small AoE. It's a medium range spell.
Enhanced Q: It has a bigger hit box and does a lot of damage initially in the early game. This WILL hurt early on against squishies, even when she doesn't build AP.
W: A tether/snare. She has to wait the same (if not more) time as Leblanc to snare a single enemy. This spell is very rarely leveled 2nd (before Q or E) and therefore generally has a very low snare time.
Enhanced W: Just makes the snare slightly longer until max rank W+R which can make it 3 seconds (but it's still a tether before it's a snare). If she's close enough to use this late game she is vulnerable to other enemies jumping on her. Yes, it heals her, but she seldom gets a ton of Hp making this not so good on a smaller health pool.
E: Her shield has the lowest base value. 190 at max level. It's meant for getting allies in or out of position moreso than being a shield.
Enhanced E: Minor Sivir ult, and a very strong shield on a single target (other targets only get half the value)
Her Ult isn't up very often in the early game. take advantage of that.