Answer:
Sometimes, for larger transactions, there are a number of things that the parties are required to do after signing. Signing simply indicates that the parties are prepared to sell and take all the necessary steps to ensure the smooth transfer of the assets/shares.
For example, first the parties sign, second the seller must sign a share transfer form and deliver this to the purchaser, the purchaser is then required to obtain finance to pay for such shares. Only when all these steps are complete will completion of the transaction take place.