so whats the point of diana?

If I remember correctly I think the goal RIOT had for her was to be a tanky bruiser/fighter, but because of her AP ratios she became more of an assassin/initiator. If she gets ahead she will easily snowball out of control, but if she falls behind she needs to rush and hope to pull off a good initiation and pray her team has enough damage.

she's like an AP irelia

bursty bruiser / tanky assassin or something along those lines

though usually built more damagey than irelia, who tends to build more tanky

Diana is actually pretty strong right now and is usually built more as an AP assassin because she can take a good amount of damage due to her shield.

I play her a bursty AP assassin/fighter, but I did her a tanky AP fighter one time against a Yasuo, and did suprisingly well, I was able to stay in long enough to out trade him and not receive as much damage, and also before I knew it I was 6-1 and he was 3-2, so I really like it, but prefer bursty assassin more.