When setting up contacts or editing existing ones, is easy to hide contact numbers. Why someone has a need to do this, only they would know. The main reason is they feel they are being spied on or watched. They may also do it if they are up to other activities they are trying to hide. In this case most would just get a disposable phone (burner). They may also do this if they know people are snooping around on their phone if they leave it unattended.