No Fees: Suica does not charge users merchant fees. Contrast that with a credit card, which charge fees as a % of transactions. Some credit cards with contactless functionality (eg. PayWave) charge a higher % if you use the contactless method rather than inserting or swiping. If you like money, well, you get more of it by using Suica. You do have to pay a 500 JPY deposit (refundable) when you first obtain the card. You can get this back at any JR station whenever you want. Also if you're using a foreign card, you might be charged foreign exchange fees and also get a bad exchange rate (ie. the rate is almost always some % worse in favor of the bank). If you can find a more efficient way of getting JPY onto a Suica, then you can save money.

Privacy: If you care about privacy, Suica is relatively anonymous, unless you (optionally) register it. It's probably possible to link a card to someone retrospectively with CCTV, but your transaction history is not being broadcast in real-time to your bank.

Acceptance: It's also increasingly widely accepted: public transport (the Tokyo metro system is amazing and gets you just about anywhere efficiently); taxis; the pervasive convenience stores + some supermarkets; more and more retail stores. Edit: as another poster pointed out, a huge % of transactions in Japan are made in cash and it's quite common for stores to not accept credit cards, yet some of these do accept Suica.

Discounts: You might get discounts on train fares if you use Suica, too.

No battery required: The SUICA works 24/7, 365, with no need to charge it. Unless I'm mistaken, smartphones require at least reserve battery for NFC payments to work.