The left sidebar has two killer features for me:
1. Finding canonical translations for technical terms. You need to know what the standard way to translate "hardware acceleration" or "differentiable manifold" is in French? Go to the wikipedia page and hover over the language switcher link in the left sidebar. Done. This has become one of my indispensable tools.
2. Learning about historical events where there are multiple inevitably biased narratives. For example, want to know about the Islamic Golden Age? The English, French, and Farsi versions have significantly different content from different perspectives (I would assume the Spanish and Arabic ones are also equally interesting). I highly recommend this exercise especially for history that one is taught in school, e.g. if you're American and can read Spanish, I bet the Spanish entry on Mexican-American War will teach you a few things you had never heard of.