For one, IP telephony is a great way of saving money. In most instances, providers will usually offer free IP-to-IP calling.
Furthermore, many VoIP phone nowadays (even entry level VoIP phones under $100) come with the G.722 codec, which means you have access to wideband audio technology. This will give you voice quality that is 2x better than what you're currently getting on your traditional phone. Of course, this does also assume that you're making an IP to IP call and that the person you're calling also has a 'high-def' phone.