The privacy of the voting booth offers substantial protections to a voter. You might threaten or bribe me, but I can vote as I please, because you can't ever know how I actually voted. Many of these protections disappear if your ballot paper is delivered to your home and returned by post.

Husbands can coerce their wives into registering for a postal ballot and then vote on their behalf. Religious leaders can coerce their followers into registering for a postal ballot and handing it over to a chosen candidate. A corrupt care home worker could register dozens of residents for postal voting, then sell their ballot papers. Migrants with a poor grasp of the English language can be tricked into giving up their ballot paper. Poor people can be offered cash for their ballot paper. Postal ballots can be stolen from postboxes and sorting offices.

All of these frauds have taken place in the UK; local election results have been nullified on more than one occasion due to endemic fraud.

http://www.telegraph.co.uk/news/uknews/law-and-order/1156001...

http://news.bbc.co.uk/1/hi/england/west_midlands/4406575.stm

http://www.bbc.co.uk/news/uk-england-london-32428648

http://news.bbc.co.uk/1/hi/england/berkshire/7302809.stm

https://www.electoralcommission.org.uk/__data/assets/pdf_fil...