We get the same data a bit faster. ISPs might need more capacity to deliver the data faster, and people may use more data - I may increase my cloud backups for example.
Personally I don't expect something for nothing. The ISPs might choose to provide this free, as an option, as another tier, not at all, or some other way. 2degrees has 100Mbps fibre for $90, 1000Mbps fiber for $106, so I'd expect this to come in at maybe $2-5 a month extra. I'd pay that, but it's not worth it to me to pay $16 a month for gigabit.