Because most all combo point trackers will use a generic stock UI function to return your combo points, and the problem is that Ferals get 2 combo points for a crit from a combo generator which takes a good second or two to resolve on the server side, which means it's just slow as fuck. This combo point tracker will watch for abilities that crit and alert you to how many you have "ahead of time".
Nearly everything in the rotation is covered here;
Nature's Swiftness availability
3 copies of Berzerk (about a 15 sec countdown to it being up, it being up, and it's duration after use)
Dream of Cenarius (the proper one for melee abilities)
3 copies of Tiger's Fury (same as berzerk; about a 5 sec countdown to it being up, it being up, and it's duration after use)
Savage Roar (2 copies; up and not up)
Rip on current target (2 copies; up and not up)
Rake on current target (2 copies, up and not up)
Weakened Armor (2 copies, up and not up -- does NOT differentiate between yours or someone elses)
A Cyclone on Focus alert
3 copies of an energy watch (under 25, above 75, and at cap;; the energy updates in weakaura's don't happen nearly often enough for it to be a great tool however**)
Nature's Vigil (2 copies, about a 15 sec countdown to it being up, and it's available)
Basically this one just watches your bleeds, procs, self buffs, any damage increases from mechanics, any attack power or mastery buffs, potion procs, everything that can affect your bleed strength, and compares it to what is already active on a target, and shows you whether a new application/refresh would tick for more based on the snapshot comparison.
In less complicated terms; it just lets you know when your bleeds will tick higher with percentages.
100 or lower is displayed as gray (eg: if it shows 80, a new bleed will be 80% as strong as your current one).
101 or higher is displayed as green (eg: if it shows 115, a new bleed will be 115% as strong as your current one)
When the number turns orange is when it would be a remarkable Damage-Per-Energy increase, though it only tends to do this when it's on the last tick or two of that bleed.
I have encountered a bug with it, where if an ability misses, it still snapshots and proceeds as normal anyway; you have been warned.
After importing it, you MAY/WILL want to reload your UI - it CAN/WILL do some pretty hefty calculations that CAN impact your FPS, but usually a reload will fix it.
Just a non-icon based display when Predator's Swiftness is active, timer included. This is most relevant to Dream of Cenarius spec, but can be useful to know when you can instant-brez someone or instant-HT yourself after an aoe damage tick.
Some other random notes:
None of the above weakaura's have extensive load settings; they will pretty much always be loaded in combat. If you are not like me and don't play only feral druids, you will want to change their load settings. The only one that this bugs me personally is with the Nature's Vigil weakaura (it will always flash at you even if you don't have that talent). But if you aren't a feral druid and these show up, that'd be annoying too; just a heads up.
No, I won't add a tracker for Incarnation. As far as PVE is concerned, you should have it macroed to berzerk, and it is fairly obvious when it is up and wears off.
No, I won't add a duration for Nature's Vigil, it's more important to use it on CD when you take that talent, and trust that you've got something around 30% uptime of it (30 sec every 1m30s = 33% uptime if used on cd).
No, I won't add a tracker for all the trinkets or profession procs/on use things. Not only do I not know what you have/use, but nearly all of them that would affect you the most with an agility proc, mastery proc, or attack power proc, will be reflected in the Hematology WeakAura appropriately. Just because you use TF+Synapse Springs doesn't mean it'll produce a higher Bleed -- especially not if your previous one had TF+NV+Pot+Trinket Proc!
Also, the only one that I use a truly custom graphic for is Blood in the Water -- the built-in radiation graphic weakaura's has pissed me off that the circle in the middle wasn't the center of the graphic (making it kind of weirdly wobble). If you want that; if it didn't import cuz I dunno if it would with an import string; I can attach it somewhere to a post here for y'all.