So the remainder of the handout agreement is there to deal with these kinds of eventualities: what to do with things that happen between signing and closing, and how to resolve confusion about whether the buyer got what they bargained for.
Conditions precedent to the closing, representations and warranties of the parties, and covenants of the buyer and seller governing behavior between the signing date and the closing date.
That will identify issues that need to get resolved: it puts the buyer in the position to make an informed decision about whether to proceed and if so on what basis. This is sort of like having an inspection before buying a house: it's a way to get disclosure of things you care about, and it gives the buyer the right to walk away from the deal if it finds things out that are not appealing.
The reps and warranties have to be true not just when the document is signed, but also on the closing date. If there's a change that makes them no longer 100% accurate, one of the conditions precedent won't be satisfied, and the buyer can say they're not going to close, or negotiate and amendment to the deal.
This gives the buyer some indemnification if it discovers, after the closing, that some rep/warranty was not true. At least in the case of a privately-held company... there's no real way to have reps/warranties survive the closing in the case of a public company. But generally with a private company they survive for a year; this is like a statute of limitations for bringing claims.
When it comes down to it, these are tools for allocating economic risk.
At closing, there's document-signing galore, and we engage in this fiction that it all happens simultaneously, as though they all get signed at the same moment. You hardly ever schedule a closing on a Friday afternoon, because you're probably going to wire-transfer the money, and you don't want a screw-up to make it last over the weekend.