IPhone: why is good to buy it on a plan?

I won't be calling often just using Wi-Fi. Should I get it on a plan or not? Also does the plan cost?

The best reason is coverage. When it comes to nationwide coverage, Verizon (and to a lesser degree, AT&T) have the best coverage. As a result, you'll be able to use your 3G and LTE in more places when you don't have access to a Wi-Fi hotspot. Also, it will give you more cell phone coverage. If you don't want a plan, then you'd probably be better off with a basic prepaid phone and an iPod Touch. Just my $0.02