Exported files are just like prints, in that they are just an output, not the original. You don't delete originals.
LR is as much a manager of your picture collection as it is an editing tool, so you can use it to keep tracks of scans of particular subjects (PS can only edit). It's also designed for editing digital captures rather than scans, so PS will have better tools for fxing things like scratches rather than the sensor dust spots that LR can handle. Also see that PS can only fix one image at a time, while with LR it's simple to sync a dust spot correction or whatever to all the other images in the sequence that had the same dust problem.
With scans, it's typical to use LR to manage the files (I assume PSD,TIF or JPEG) and then choose Edit in PS to send them for all PS's various pixel editing tools. Save in PS, and those edits are shown in LR.