This is about how I get a brand new out of the box belt sander ready for work, so that it produces as good a job as possible. Inevitably the bed on these machines isn't perfectly flat, so we lap these to get them as near perfect as we can. Also, the rollers are always slightly lower than the bed itself, which causes a bit of "run-off" at the ends of the work - in order to correct this I build up the rollers with several layers of gaffa tape. This needs to be done carefully to avoid seams and to ensure a near as perfect a finished surface on the rollers as possible.
This isn't a problem with this particle brand, it seems to apply to all the makes of sander that we've used - Machine Mart (Clarke), Record, Ferrex (Aldi) and the Lumberjack that is shown in the video.
Of course you can use these machines straight out of the box and they will work OK, but if you want a bit more accuracy, a couple of hours preparation work will pay dividends in the long term.