The idea
This is to enact a blanket principle, that simply states: in all law concerning occupancy of your home, as owner or tenant, every fact of law or point of law that is true shall be absolutely true with no shades of uncertainty, and shall disprove that its opposite is true.
This is to prevent manipulation and swindling by your solicitor. At present, there can be all sorts of complications arising from planning issues, certifying the completion of extensions, or "burdens", duties attached to a piece of land for murky reasons of its history. When these complications or any others arise, you are left to rely on your solicitor's opinion, with no definite reference point to say it is right or wrong, and solicitors are allowed to have different conflicting opinions on the same thing yet both count as right. This is manipulation, it leaves you with no entitlement to know for certain what the law is that you are trying to comply with.
These things are true in both the English and Scottish systems.
Why is it important?
It leaves you dangerously unprotected if your solicitor does a bad job: oh it was just his opinion and professional judgement, the Law Societies of both countries will always say. Conveyancing is supposed to be about protecting you from all legal probelms that can arise from your occupancy, but when you actually have a problem with the way it was done, you find that the protection is never expected to be total, only a matter of judgment. Especially with planning and completion of works, some of this referring to issues that are simply not reckoned to have any single certain answer at all. That allows swindles to happen. Solicitor can advise you it is necessary to make a deal incurring expense that s/he judges will likely be recoverable, then change the position to be that no it's not recoverable after all, but that was just my judgment in working on my client's problem, the Law Society won't give you any redress for it.
If there is no single definite certain position in what the law is, then there is no law. That is common sense. Either you are in a position or you are not. The present situation, still an unrefomed relic of customs developed at random in the Middle Ages is unconstitutional because it gives you no certain law you can defend your actions by or protect yourself by, and it forces you to have your interests and assets cared for by unaccountable gameplayers who can treat you arbitrarily, which is more like a reign of terror than protection. What solicitors dealing with housing deal with at present is a chaotic mess, and leaving it like that makes the job harder for everyone, them too.
The principle that a fact of law "disproves its own opposite", disproves that its opposite can also be a fact, is necessary to enact, to stop solicitors taking conflicting views on the same item and being allowed both to count as right.