Pearl diver vs nothing is pretty easy to explain. You have an ~3.8% chance of rescuing your power five from the bottom of the deck (5/12 for Pdiver being in T3 and 1/11 for the five to be bottom decked as we will always play Pdiver and not care if it misses the shuffle). Without draw, Pdiver effectively a zero space card so you still only have a base 1/11 chance of missing the 5 on the first shuffle (this is the same benefit of 5/nothing). That 1/11 chance of your 5 being bottom decked represents the single biggest draw luck that can nerf a power 5 opening, when you bottom deck a Mountebank you have effectively opened 0/0/5 and gotten lucky on the 5. I could be wrong, but when I think missing the power 5 in the reshuffle is the biggest reason why dominant openings lose.
The other big thing is that Pdiver openings tell you about the other 2's. Think about the power 2's. Most of them actually hurt power 5 openings - Lighthouse makes it easy to block devastating power 5 attacks. Chapel heavily mitigates not just the cursing attacks, but can also stop up the discard attacks if there are also cards like Watchtower, Library, Menage, etc. out. Squire can allow you to quickly slingshot up to a power 5 attack (mitigating the advantage of opening 5/2) and make trashing attacks like the Knights risky. Even Moat can reduce the power of attacks (e.g. Council room/Ghost ship is nowhere near as good with Moat in the kingdom) a good bit. Chapel/Mountebank is stronger than Pdiver/Mntbnk, but opening Mntbnk is much stronger when trashing isn't in the kingdom.
Buying Pdiver for 2 signals that there wasn't a better 2 out and the lack of such power 2's often means the Mountebank or whatever is that much stronger. Even outside of the 2's, Pdiver takes up a kingdom slot and signals that there are lower odds of some other strategy being dominant. Witch/nothing is still quite strong, but Pdiver/Witch signals that you have lower odds of having Masquerade, Ambassador, or some other hard counter in the kingdom.
And of course there are all the other ways Pdiver can actually be directly beneficial, people who open something like Margrave/Pdiver are much more likely to have any of the following out than Margrave/nothing:
1. Using it to activate cards like Conspirator, Horn of plenty, or Peddler.
2. Using it to help diversophiles like Menage or Harvest.
3. Using it with Tr/Kc/Prssn/Golem to gain action balance.
4. Using it with an action-only Tfb like Graverobber or Prssn.
5. Using it go gain top deck knowledge (e.g. Native Village) or when you have top deck knowledge (e.g. Apothecary).
Pdiver is normally easily better than nothing with a power 5 - it has a non-negligible chance of saving a missed shuffle and it has enough possible uses that the winning rate may more reflect boards where Pdiver is additionally useful rather than just being cantrip.