Q:

Why is Spain's base timezone UTC+1?

Looking at the time zone map on Wikipedia we can see that countries such as France and Spain technically should have a time zone of UTC+0. Wikipedia says:

France previously used GMT, but was switched to CET (Central European
  Time) during the German occupation of the country during World War II
  and did not switch back after the war.[15] Similarly, prior to World
  War II, the Netherlands observed "Amsterdam Time", which was twenty
  minutes ahead of Greenwich Mean Time. They were obliged to follow
  German time during the war, and kept it thereafter

But Spain was not occupied by Germany… How did it happen that their time zone is UTC+1 then?

A:

In 1940, Spain changed its time zone from GMT to GMT+1, as Franco thought that it would be a good idea to have the same time as Nazi Germany and Fascist Italy (his political allies) after Germany occupied France. The United Kingdom modified its time zone too, but reverted its decision in 1945.
In the 80s, the PSOE (political party at government) institutionalized the custom of adding another hour in the summer, i.e., the time gap is more than two hours for more than six months every year.
In October 2012, in the VII National Congress for Rationalization of Spanish Time Zones (remember that there are two time zones in Spain, one for the Peninsula and the other for the Canary Island), Nuria Chinchilla, has proposed to return to GMT.

A:

Primarily for convenience of trade and communications across national borders. As the countries of Western Europe have become ever more closely linked, it makes life easier if people can agree on what time it is.
The initial standardisation of times, in Great Britain at least, came with the railway - Bristol time was 11 minutes different to London, based on the time of sunrise, sunset etc, and this played merry hell with timetabling.
As communication and transport have improved, it has become important to co-ordinate times across a wider area. Even up to the 1930s few people would drive from Spain to France, and transport links were fairly poor.
Having said that, I'm not sure why the same argument would not lead Portugal to join the movement towards GMT + 1, so it clearly isn't an all-powerful line of reasoning.

