There are greater constraints on the complexity and length of the questionnaire: if it is not short, simple and easy to complete the respondent is likely to bin it. A stamped addressed envelope or a freepost number can be used to facilitate return.
Source: wiktionary
There is a belief that real postage stamps lead to a greater response. This seems to be because respondents see the stamp as money and are less likely to throw it away; in comparison, a freepost or pre-paid envelope is worthless.
Source: wiktionary
Freepost on one election communication
Source: wiktionary