Your contact's Overview screen contains a Primary Contact section for the contact within the Know Your Client area. It can be accessed by clicking the Misc Client Info tab in that area.
Below is an example of this section:
As you can see, you can enter a Name, Email Address and Phone number for the contact's Assistant or other Primary Contact.
You can edit this information by clicking "Edit" in the upper right corner of the Primary Contact widget or delete them by clicking "Delete".
In regards to Mail Merges, this section can prove especially useful for entering contact names for Contact Record Types other than Individual (Business, Trust, Union, or Association). The Primary Contact Name, Phone Number and Email Address are acceptable Mail Merge Fields, thus you can pull them into merged templates, provided you've inserted the correct fields to do so.