Those above gave good explanations so not sure what I can add, but to say it's worth it, to me using Pro.
Many users swap ideas about how to make their own ideal piano sounds from the defaults. If you don't wish to delve into too much detail, just moving the 'Condition' slider to the right may add a variable amount of 'randomness' into the engine which will be satisfactory (just making the piano seem not so perfectly set up - to me, this quickly adds 'swimming' to the sound which is instant and pretty effective often).
But, you certainly can do many things to alter more exacting things, for any piano/preset, to make it more the way you desire.
You could for example, right-click on the 'Condition' slider and choose to change the 'Random seed' number. I've spent time in the past entering different numbers with the slider far to the right to hear what excessive alterations occur, and noted down a few numbers which seemed to do quite different things. When used normally, some DO seem to make the piano seem reminiscent of the extreme sound. If an extreme setting is particularly sour, or strangely not so sour.. note those numbers to re-use as random seeds. (I lost my notes on another PC.. maybe will be able to find those in future.. but I'll probably do this over again sometime soon).
Some may like just a little, or quite a lot of change when using the Condition slider. I do enjoy sometimes adding some of that slider, even if I'd delved into a pretty densely edited preset.. normally something like 0.06 to 0.18 (that's my own personal enjoyable 'realistic' bubble I tend to stick to.. others may like more, or less).
In Pro though, for 'humanizing' exact items.. it's certainly best to think "Do I need to do it" and if you think your pianos could be made more like you'd wish to hear them there, then you may decide Pro is a good expenditure just for this.
Most of the randomizable controls already contain a tasteful amount of randomization. But it's often good to be able to add, or reduce, certain things if you feel that it may improve your piano's expressiveness. It's interesting getting to know different controls and hearing what they each do.. I'd like to suggest, try out lowest vs. highest settings, no matter which control.. don't be scared to push from zero to max and back.. and then when you know "Ah, so that's what this does".. then stay within the zone where it sounds realilstic, if that's your goal.. small changes to anything can amount to pretty large alterations in realism.. so it may make sense to make something sound much sweeter.. but is that realistic for this control... and in time you notice ways to use 2 or more controls more subtly to gain better realism than just altering 1 control a lot.
If using Pro, you can easily try this to hear how the randomizing tool works in the controls..
In "Voicing" in the main interface, find "Hammer hardness" settings in the middle there.. then right-click the "Piano" hammer slider, and you'll see a panel with the randomization adjustment tool. If you do only this.. drag the bottom dot far to the right side and then you'll notice the 'hill-shaped' area is now stretched all the way right.. that's representing the distribution of probabilities (that you may strike a 'softest' note, but hear a hard hammer doing so, instead of a hammer at normal 'softness/hardness').
If you use the mouse to tap exactly the same soft notes (by tapping at the top of the piano keyboar representation), you'll hear the notes played with normal and some brighter sounding tones.
That gives just one example of some obvious jumps in tone - but often you can use the randomizations to make something more exact with less variance.
In real life, even a solidly built new piano may have very tiny variations with each key strike. So, it probably is more realistic not to make wild variations like random 'hard hammer' sounds for softly played notes - that would indicate a damaged hammer with some felt missing or a cartoon mouse living in the piano making mischief
The main thing I'd suggest if using ramdomized settings (if realism counts), would be to make many, but quite small adjustments to each. I do think it may be tempting to go with a higher random number for some element - but over time, you may return to that and after novelty fades, you realize "Oh, that's cool, but not really realistic".. but this IS actually the fantastic extra thing about Pianoteq - go wild if you want, and make infinite 'new' instruments.. there really is no rules for that, and I do love making 'unknown' instruments replacing synthesizers etc.
IDEA @Philippe: Maybe in future, some controls I imagine will go beyond 'randomization' and perhaps could be bracketed into factors such as "crescendo" (all randomness increases by X% when more than X strings played within X seconds over X velocity - and perhaps other things?).
All these little things are part of where reality lives and I'd definitely encourage upgrading to Pro for this ability, although Pro gives so much more also.
Pianoteq Studio Bundle (Pro plus all instruments) - Kawai MP11 digital piano - Yamaha HS8 monitors