Hi,
None would mean no HTTP Profile is assigned to the virtual server. This means the BIG-IP would have no concept of Layer 7 / HTTP traffic on this VIP. So, if you needed any HTTP type stuff, or manipulation, for example to insert cookies, cookie persistence, header modification, irules to change the URIs - any of these you would not be able to do without an HTTP profile assigned.
Whilst the default HTTP profile will suffice most often, i would still recommend creating a custom one, built upon the default one, even if you accept the same settings. That way you have more control over the individual elements - it is not best practice/recommended to amend the default HTTP profile.
Hope this helps,
N