The position of Notaries in Spain is different to what you may be used to in the UK.
In they are usually associated with Powers of Attorney, however their role in Spain is much more wide ranging and they have more powers that include verifying legal documents
In short, they have the last word in any major legal transaction that requires approval and ultimately is a public record. They do not represent individual clients but are the last link in any legal process to be able to verify that transactions are carried out legally and correctly with all supporting documents matching up and scrutinized by the Notary .
The link below explains this in much more detail so that you are aware of the reasons why property purchases/sales and Wills involve notarisation.