If you simply set MANPATH, it overrides the default and you lose access to the standard man pages. For example, man ls works before setting MANPATH, but does not work afterwards.

To append a search directory without overriding the default, prefix a colon to MANPATH like this:

export MANPATH=":/path/to/custom/man"

Adding the colon gives you access to both the standard system man pages and the custom pages referenced in the MANPATH variable.

This answer brought to you by manpath(1):

If $MANPATH is set, manpath displays its value rather than
determining it on the fly. If $MANPATH is prefixed by a colon, then
the value of the variable is appended to the list determined
from the content of the configuration files. If the colon comes at
the end of the value in the variable, then the determined list is
appended to the content of the variable. If the value of the
variable contains a double colon (::), then the determined list is
inserted in the middle of the value, between the two colons.