[wongarsu]

It's called defence in depth. If you can prevent 99% of malware infections and can prevent 75% of malware from phoning home, you have a 99.75% confidence (1 in 400) of not having a data leak due to a compromised machine. That's 4 times better than only preventing infections.

[TeMPOraL]

I'm pretty sure the real reason is managers not wanting their employees to do private browsing while at work.