Why do you need your own NTP server although there are NTP servers available on the internet?

If an internet connection is working properly then NTP can determine and account for the packet transmission delays quite reliable. However, if the internet connection is at its capacity limit, time synchronization can be significantly degraded due to high dispersion in packet transmission delays. Reasons for this may be hacker attacks, which must not address your own network, or new viruses causing a huge flood of emails, like it has already happened in the past. An own NTP server cannot easily be compromised out of the internet.