Signing a reservation contract is not so common in other countries,
But it became more common in Spain during the real estate boom, as houses were selling very quickly and it was necessary to reserve the property while the papers and mortgage were gathered so that no one else could buy it in the meantime.

It is a contract between the buyer and the seller of a property (usually made through the agency) that commits the buyer to buy within a specified period by paying a reservation fee (normally between 5 and 15% of the value of the property).
If the buyer changes his mind, he will lose the amount paid.
But if the seller changes his mind (he sells to someone else) he will have to pay double the amount paid to the buyer.

The contract is important because it reflects the conditions of the future transmission.

For this reason, the deposit contract must collect the information that will later be contained in the sale contract.

The minimum information that must be included in the contract is:

Personal data of the selling and buying party.
Description and identification of the property.
Final price of the sale and payment method.
Amount of money of the signal or advance (which will be deducted from the final price).
Maximum time to formalize the sale contract.
Commitment to sign the contract by public deed.
Distribution of the possible expenses of the sale.
Signature of buyer and seller.

This type of contract is still being used as it makes it easier for the seller to commit the buyer to more serious deals.

You don't have to sign a reservation contract, but the possibility of losing the deal is still there.

Keep in mind that properties are often advertised at multiple agencies and another agency may sign a contract with its client while you may be negotiating the mortgage.

Now our advice is to reserve the property if you are really interested.