Registration with the Spanish consulate is not optional. It is a legal obligation for Spanish nationals living abroad, although in practice nobody will come looking for you if you fail to do so. The problem is what you miss out on when you are not registered.
The Consular Registration Register (REMA) is the basis on which almost all administrative matters relating to Spain are handled when you live abroad. Without this registration, you cannot apply to vote in Spanish elections from Switzerland (the ‘voto rogado’ system), you will not appear on the Electoral Register for Absent Residents and, in practice, you forfeit your right to participate in Spanish political life. Nor do you have standard access to certain consular procedures, such as renewing your DNI or passport under normal circumstances.
In Switzerland, there are consulates in Bern, Geneva and Zurich. Registration is free of charge and can be done in person or, in part, online via the Ministry of Foreign Affairs’ website. You must present a valid passport and proof of residence in Switzerland: either your residence permit or confirmation of your address from the cantonal authorities. Some consulates require you to book an appointment in advance, whilst others offer part of the process online. It is advisable to check this before you go.
One thing many people realise too late: being registered with REMA does not affect your Swiss residence permit or any administrative obligations here in any way. They are two completely separate registers. Registering does not mean declaring that you live in Spain, but exactly the opposite: that you are permanently resident abroad. And far too many Spaniards only realise this at the worst possible moment, when they need to carry out an urgent administrative procedure and it turns out they aren’t registered anywhere.