In modern French elections, electors usually cast their votes by placing a pre-marked ballot paper with the name of their desired candidate into an envelope, and depositing this in a ballot box. In most of the western world, the practice instead is for voters to be given a list of candidates on a single sheet of paper, and to mark (or number) the candidate(s) of their choice. What is the origin of this French voting tradition, and has France ever considered moving to the method of balloting used in most of the world?
You might be interested in this SI web exhibit. It notes that the United States in the nineteenth century largely used the system of pre-printed ballots (this is the origin of the term "party ticket"), but moved on to lever machines and other systems. So, it seems like the French system is the anachronism rather than the innovation.
So the question is perhaps better asked as "Why do the French continue the outmoded practice of submitting pre-marked ballots, rather than a more modern system that allows greater voter choice?"
I can't answer that one, though.
It's considered a way to prevent fraud: any balot with even a single mark is considered void (not all local elections are like that though). Counting is public: anyone can volountier to count and anyone can go to the counting office to watch over the counters' shoulder to ensure it's done correctly. If it was allowed to put a mark on a ballot, a candidate could say "I'll pay you to vote for me. Mark your ballot wih this sign so I recognize it." The goal is than any citizen can make sure votes are counted correctly without being able to prove to someone else what they voted.
As /u/pipkin42 said, the French way is the "old" one. There are more and more pushes toward electronic voting, but no other method has yot been considered to give the same level of guaranties.