CatThe suits of tarot are based on the four elements. In classical systems, earth and water are paired as the "heavy" elements, whereas air and fire are the lighter or "higher" elements.
Think of it like Pokémon, where some elements are strong or weak to each other. 
Taking these pairings into account can help lend more clarity to multi-card readings.
This is illustrated perfectly in the Rider-Waite-Smith deck. If you pull out all the suits of the same number, you see how the images on the Cups-Pentacles and the Wands-Swords reflect each other.
