Why would the falcon heavy sit on the launch pad if it is not yet ready to be launched? Wouldn't this just make it susceptible to damage to inclement weather and possible (although not probable due to lightening rods) lightening strikes?
This is a new booster (three cores vs one is a big change). The TEL was updated to hold it. Clearly it 'fits' on the TEL, since they mounted it on the TEL in the HIF.
But then they raised it at the pad to make sure all the connections are where they need to be placed, and that they all fit.
Next up is the wet dress rehearsal (WDR) in which they then actually fill the boosters up with fuel and oxidizer to ensure that they can handle the load (enough prechilling for all the LOX and RP1).
Then they drain it (which is also a test, since if they have to abort they need to be able to drain the tanks safely) and roll it back into the HIF to ensure all is still well.
If, however, the WDR goes well they are suggesting that they would go straight into a static fire, since they are all dressed up with nowhere to go. If not, they will fix whatever the issue requires.
Then, finally, they will get ready for a real launch.
It is possible, likely even, that the payload will be removed for the WDR/Static fire as a holdover from the AMOS-6 RUD. Or not. Time will tell.
Because this will be the first Falcon Heavy launch, they did a fit check: basically they ran through the process of moving the rocket to the launch pad and erecting it, to see if everything works as designed, a.k.a. validation.
The rocket will be rolled out again for a static fire (i.e. running the first stage engines while holding the rocket down): another test for rocket, pad and prelaunch procedures.
When that's done, there will be a final inspection (vehicle will be rolled back into the hangar for this) and then it's time to launch.
Sometimes a separate, non-flight rocket is used for fit checks. There were 5 non-flight Saturn V rockets, for instance. While this reduces the risk to flight hardware, it's an expensive way to work.
Risk to the hardware from weather can be mitigated by using the same procedures as for a launch: the weather forecast is checked, and if weather is outside of preset parameters, the rollout doesn't take place.