Unless you get visits from loads of teenage relatives, all wanting to stream HD movies simultaneously, it's unlikely that it makes any difference at all as to whether you get 15 Mbps or 15 million Mbps; you're unlikely to notice any difference.
I've still got a copper-cabled ADSL2+ connection, which pre-dates fibre ones. I get around 10 Mbps, which is perfectly adequate for (say) streaming NOW TV and browsing the web simultaneously. A connection offering tens, hundreds or even thousands times my current speed wouldn't really make any difference to me - except for the bigger hole in my bank balance!
Don't be taken in by BT, Virgin Media, et al, who all want you to think that 'faster' must always be synonymous with 'better'. It simply isn't.