Following text is taken from the manual page of systemd.unit (man systemd.unit). 

 WantedBy=
           Installs a symlink in the .wants/ subdirectory for a unit. This has the effect that when the listed unit name is activated the unit listing it is activated too.  WantedBy=foo.service in a service bar.service is mostly equivalent to Alias=foo.service.wants/bar.service in the same file.

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I believe it is not so clear to say, "Installs a symlink in the .wants/ subdirectory for a unit." By just editing this file, no symbolic link is created. It might have been better to say:
"A symbolic link is created in the .wants/ folder when the unit is activated by <systemctl enable>. 

I also tried to re-phrase the second sentenece:
When the unit assigned to "WantedBy=" is activated, the unit having "WantedBy=" is activated too. If bar.service unit has WantedBy=foo.service, it can be read as "foo.service wants bar.service".