The following remarks are aimed at Spain’s foreign residents. 470,000 Europeans and a few other countries with bilateral agreements, including, for some reason, New Zealand and Trinidad and Tobago, have the right to vote in the municipal elections, a few less – 366,000 – in the European ones. 97,585 Brits can vote in the local elections, being both on the padrón and having duly claimed their right to suffrage.
You can vote in local elections if you are on the voting list (which many of us aren't - well done that man!). In the smaller municipalities where most foreigners live, our vote does make a difference. A town hall can be won or lost on just a few votes... maybe even just the one.
Of course, there are those who can vote but won't because 'it's not our country', or 'we are guests here' (Oh yeah? Where's your invitation?) or 'we don't know the issues'. Normally, the voters do know the candidates, or at least some people on their list; and in small communities, it's not about party politics so much as personality politics. There are good socialist mayors; there are good conservative mayors too.
Some local politicians are there for the opportunities. Madrid has long attempted to curtail their powers, but local mayors and their senior councillors often leave politics far richer than when they arrived. They are active in preparing the local urban plan (which will often feature land they have recently acquired, or land that receives a favourable consideration following a short but intense talk with the owner). There are commissions to be won for putting up new street furniture or proving jobs or choosing one supplier over another. Sometimes this activity doesn't matter so much in the Great Scheme of things, other times it does.
Not all local politics is about the monetary opportunities. Some others want to climb from their municipality into provincial, regional or even national politics. That's ambition.
Other local politicians are genuine hard-working people who sometimes even forego their salary. They may have given their time and energy to help their communities. Feel proud of them, as many will cross the street to avoid them, or sometimes plot their downfall by buying one of their councillors and calling for a Vote of No Confidence.
Voting is important, because it makes it your responsibility too when the town does well.
As for the European elections, where many of us can vote as well, the question boils down to ‘for who?’ We know none of the candidates (and they most certainly will never speak for us in Brussels). While EU parties remain mono-national, this will be the case. An exception to this state of affairs is Volt Europa which wants ‘to create the first transnational party in the European Parliament’. One day.
Meanwhile, an article here says that the European Parliament is controlled by the lobbies rather than the 400 million voters. Who knew?
Remember also, that four years down the line and following the implosion of the UK thanks to Brexit, the chances are good that we Brits could lose our vote in local elections in Spain (to say nothing of the European vote), despite the recently signed bilateral agreement between Madrid and London, guaranteeing the voting rights of British nationals in Spain and Spanish nationals in the UK to be protected and written into law by both countries.