Is it really necessary to sign over a Power of Attorney?

You do not have to sign a POA but it is strongly advisable to do so. You may believe that you will be able to travel to sign the deeds for your new property, but there is always the unforeseen: Your money may not arrive in time, your plane may be delayed, the vendor may have problems arriving at the Notary, the Notary may be off sick, there are numerous reasons why the signing might not take place on the agreed date. If your lawyer has a power of attorney, then they can reschedule the signing. The POA will also allow them to apply for your NIE number, check the property register, register the property in the Land Registry, pay taxes, open a bank account, and deal with the utility companies.