Taking a taxi in Paris is not necessarily the most convenient way to travel. They’re relatively expensive, sometimes hard to find, and not all taxi drivers know the way in France’s biggest city. If you’re not clear about your destination, it’s quite possible that you’ll be in transit longer than you want.
It is nice however, to take a taxi back to your hotel after a long day of walking, or a long night of partying. Some people aren’t comfortable riding metros or buses at night, so the taxi is always a safe option.