My very first deck was the Sacred Rose, I bought the Thoth soonafter as well. Some time passed, and I got the Golden Klimt - just because it was there, in the store, and I liked it. I never used the Thoth or the Klimt, only the Sacred Rose for years and years on end.
Then, something cracked in me - when I purchased Haindl Tarot, I knew I was done - I was going to buy lots more decks. Soonafter, I was travelling in the North of Europe and bought some decks.
I collect because I love to collect, period. Why Tarot cards specifically? I love to flip through them, have them near, look at the imagery and study new things. Want to know about life in the circus? Zirkus Mägi! Want to learn about celtic culture? Druidcraft! Native American tribal culture? Vision Quest! Mythology? Apokalypsis! Wonderful study opportunities lie all around you. Plus, all decks I buy are very pretty to behold.