You might want some redundancy in case the bank accidentally destroys your safety deposit box [1] or the bank is raided and valuables are confiscated from every single deposit box [2].

[1] http://www.dbs.com.hk/en/aboutus/newsroom/2004/041005/

[2] http://www.dailymail.co.uk/news/article-1362561/Bank-gangste...