No I don't either but I am assuming that there are some who do otherwise why would these offers be made? Presumably research shows that they work?
It's an incentive. If you are suggesting people sign up to an ISP only because of a voucher offer then no, I don't think that happens. Why would anyone ignore the fact broadband costs £50 a month and sign up just because of a £75 voucher? That would be ridiculous.
However, I do believe if someone is interested in an ISP anyway
, the voucher offer may "push them over the edge".